KEY TO PALAEOGEOGRAPHICAL MAPS A clear distinction has been made between factual evidence, shown largely in the form of patterns representing lithofacies, confined to the immediate vicinity of outcrop or borehole data, and palaeogeographical interpretation, shown largely as areas of colour tints. Many of the earlier maps are drawn for times when the various suspect terranes which make up Britain and Ireland demonstrably did not have the'configuration they have today. Although spatial parameters are wanting in many cases, an attempt has been made to convey something of the geography of these times.
Atlas of Palaeogeography and Lithofacies
Dedicated to the m e m o r y o f the late Professor L.J. Wills H o n o r a r y Fellow o f the Geological Society Lyell medallist and Wollaston medallist A u t h o r o f ,4 Palaeogeographical Atlas o f the British Isles, the inspiration for this volume
References to this volume It is recommended that reference to all or part of this Atlas should be made in one of the following ways.
COPE, J. C. W., INGHAM, J. K. & RAWSON, P. F. (eds) 1992. Atlas of Palaeogeography and Lithofacies. Geological Society, London, Memoir, 13. HANCOCK, J. M. & RAWSON,P. F. 1992. Cretaceous. In: COPE, J. C. W., INGHAM, J. K. & RAWSON, P. F. (eds) Atlas of Palaeogeography and Lithofacies. Geological Society, London, Memoir, 13, 131-139. SWAN, A. R. H. 1992. Namurian. In: COPE, J. C. W., INGHAM,J. K. & RAWSON,P. F. (eds) Atlas of Palaeogeography and Lithofacies. Geological Society, London, Memoir, 13, 78-80.
GEOLOGICAL SOCIETY MEMOIR NO 13
Atlas of Palaeogeography and Lithofacies E D I T E D BY
J. C. W. COPE, J. K. I N G H A M
& P. F. R A W S O N
1992 Published by The Geological Society London
THE GEOLOGICAL
SOCIETY
The Geological Society of London was founded in 1807 and is the oldest geological society in the world. It received its Royal Charter in 1825 for the purpose of 'investigating the mineral structure of the Earth' and is now Britain's national society for geology. Both a learned society and a professional body, the Geological Society is recognized by the Department of Trade and Industry (DTI) as the chartering authority for geoscience, able to award Chartered Geologist status upon appropriately qualified Fellows. The Society has a membership of 8600, of whom about 1500 live outside the UK. Fellowship of the Society is open to persons holding a recognized honours degree in geology or a cognate subject and who have at least two years' relevant postgraduate experience, or not less than six years' relevant experience in geology or a cognat subject. A Fellow with a minimum of five years' relevant postgraduate experience in the practice of geology may apply for chartered status. Successful applicants are entitled to use the desiignatory postnominal CGeol (Chartered Geologist). Fellows of the Society may use the letter FGS. Other grades of membership are available to members not yet qualifying for Fellowship. The Society has its own Publishing House based in Bath, UK. It produces the Society's international journals, books and maps, and is the European distributor for publications of the American Association of Petroleum Geologists (AAPG), the Society for Sedimentary Geology (SEPM) and the Geological Society of America (GSA). Members of the Society can buy books at considerable discounts. The Publishing House has an online bookshop (http://bookshop.geolsoc.org.uk). Further information on Society membership may be obtained from the Membership Services Manager, The Geological Society, Burlington House, Piccadilly, London W1V 0JU (Email:
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First published 1992; revised reprint 1999 The publishers make no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility for any errors or omissions that may be made. 9 The Geological Society of London 1992. All rights reserved. No reproduction, copy or transmission of this publication may be made without written permission. No paragraph of this publication may be reproduced, copied or transmitted save with the provisions of the Copyright Licensing Agency, 90 Tottenham Court Road, London W1P 9HE. Users registered with the Copyright Clearance Center, 27 Congress Street, Salem, MA 01970, USA: the item-fee code for this publication is 0435-4052/99/$15.00 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. ISBN 1-86239-055-X ISSN 0435-4052 Printed by The Alden Press, Oxford
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Contents vii ix x xi xiii
List of Contributors Editorial Editorial to the revised edition Introduction Key
Terranes
1
T 1: Terranes
3
Precambrian
5 7 7
C3: C4: C5: C6:
C7: C8: C9:
Latest Courceyan-early Chadian Arundian Brigantian Namurian: Arnsbergian Namurian: Marsdenian Westphalian A (Langsettian) Westphalian D
Permian PCla: PClb: P4~2:
Upper Proterozoic Stoer Group Upper Proterozoic Torridon Group Northwestern British Isles: Upper Proterozoic Dalradian Supergroup PC3a,b: Southern British Isles: late Precambrian
Cambrian Cla: Clb: C2a: 4~2b:
Southern Southern Southern Northern
British British British British
Isles: Isles: Isles: Isles:
Comley St David's Merioneth Comley
Ordovician Ola: Olb: O2a: O2b: O3a: O3b: O4a: O4b: O5a: O5b: O6a: O6b: O7a: O7b: O8a: O8b:
Northern British Northern British Northern British Northern British Northern British Northern British Southern British Southern British Southern British Southern British Southern British Southern British Southern British Woolstonian) Southern British Southern British Southern British
Isles: Tremadoc Isles: mid Arenig Isles: late Llanvirn Isles: mid Llandeilo Isles: mid Caradoc Isles: mid Ashgill Isles: Tremadoc Isles: Arenig Isles: late Llanvirn Isles: mid Llandeilo Isles: Caradoc (Costonian) Isles: Caradoc (Harnagian) Isles: Caradoc (Longvillian including Isles: Caradoc (Onnian) Isles: mid Ashgill (Rawtheyan) Isles: late Ashgill (Hirnantian)
Silurian Sla: Slb: S2a: S2b: S3a: S3b: S4a: S4b: S5a: S5b: S6a: S6b: $7: S8a: S8b: $9:
Southern British Isles: latest Ashgill-early Llandovery Southern British Isles: Llandovery (Aeronian) Southern British Isles: Llandovery (Aeronian) Southern British Isles: (Telychian) Southern British Isles: Wenlock (Sheinwoodian) Southern British Isles: Wenlock (Homerian) Southern British Isles: Ludlow (mid Gorstian) Southern British Isles: Ludlow (late Gorstian) Southern British Isles: (early Ludfordian) Southern British Isles: (late Ludfordian) Northern British Isles: Llandovery (Rhuddanian) Northern British Isles: Llandovery (Aeronian) Northern British Isles: Llandovery (Telychian) Northern British Isles: Wenlock (Sheinwoodian) Southern British Isles: earliest Pfidoli All British Isles: mid Pfidoll
9 11 13 15 15 17 17 19 21 21 23 23 25 25 27 27 29 29 31 31 33 33 35 35 37 39 39 41 41 43 43 45 45 47 47 49 49 51 53 53 55
Devonian
57
D 1: D2: D3: D4:
59 61 63 65
Lochkovian Pragian-Emsian boundary Givetian Frasnian-early Famennian
P 1: P2a: P2b: P3a: P3b: P4a: P4b: P4c: P4d:
87 Early Permian Lower part of English Zechstein Cycle 1 and equivalents Upper part of English Zechstein Cycle 1 and equivalents Lower part of English Zechstein Cycle 2 and equivalents Upper part of English Zechstein Cycle 2 Lower part of English Zechstein Cycle 3 and equivalents Upper part of English Zechstein Cycle 3 and equivalents Upper middle part of English Zechstein Cycle 4 and equivalents English Zechstein Cycle 5 and equivalents
Triassic Trla: Trlb: Tr2: Tr3a: Tr3b: Tr4a: Tr4b: Tr4c:
J10: Jl la-d:
Late Ryazanian (Berriasian) Mid Hauterivian Late Aptian Latest Albian Early Cenomanian Late Campanian
Palaeogene and Neogene Pg 1: Pg2a: Pg2b: Pg2c: Pg2d: Pg3: Ng 1:
Palaeocene Early Palaeocene and the sub-Palaeocene floor Early Eocene Mid Eocene Late Eocene Mid-late Oligocene Miocene-Pliocene
Quaternary Carboniferous CI: Devonian-Carboniferous boundary (latest FamennianC2:
earliest Courceyan) Mid Courceyan
67 69 71
Qla: QI b: Q2:
95 95
99 99 101 103 103 105 105 105
107 Early Hettangian Early Pliensbachian (Carixian) Late Pliensbachian (Domerian) Mid Toarcian Early Aalenian Late Aalenian Early Bajocian Late Bajocian Mid Bathonian Late Bathonian Early Callovian Mid Callovian Mid Oxfordian Late Kimmeridgian (early Volgian) Portlandian
Cretaceous K 1: K2a: K2b: K3: K4a: K4b:
89 91 91 93 93 95 95
97 Early to mid Sythian Earliest Anisian to earliest Carnian Earliest Anisian to earliest Carnian Earliest Anisian to earliest Carnian Latest Norian to earliest Rhaetian Mid to late Rhaetian Late Carnian (detail) Late Rhaetian (detail)
Jurassic J 1: J2a: J2b: J3: J4a: J4b: J5: J6a: J6b: J7a: J7b: J8: J9:
73 75 77 79 81 83 85
109 111 111 113 115 115 117 119 119 121 121 123 125 127 129 131 133 135 135 137 139 139 141 143 145 145 145 145 147 147
149
Quaternary geography 151 Quaternary crustal movement 151 Present-day seabed sediment distribution and topography 153
List of Contributors R. Anderton, formerly Department of Applied Geology, University of Strathclyde (now BP Glasgow)
C.P. Hughes, Department of Earth Sciences, Sedgwick Museum, Downing Street, Cambridge, CB2 3EQ
M. G. Bassett, Department of Geology, National Museum of Wales, Cardiff, CFI 3NP
J.K. Ingham, Hunterian Museum, The University of Glasgow, Glasgow, GI2 8QQ
R. E. Bevins, Department of Geology, National Museum of Wales, Cardiff, CF1 3NP
H. C. lvimey-Cook*, British Geological Survey, Nicker Hill, Keyworth,
B. J. Bluck, Department of Geology and Applied Geology, The University of Glasgow, Glasgow, GI2 8QQ
J.D. Lawson, Department of Geology and Applied Geology, The University of Glasgow, Glasgow, Gi2 8QQ
M. J. Bradshaw, formerly Department of Geology, University of Aston (now Brunei Shell Petroleum Sendirian Berhad, Seria, Negara Brunei Darussalam, Borneo)
J. W. Murray, Department of Geology, The University, Southampton, SO9 5NH
M. D. Brasier, Department of Earth Sciences, The University, Parks Road, Oxford, OX1 3PR P. J. Brenchley, Department of Earth Sciences, PO Box 147, The University, Liverpool, L69 3BX
Nottingham, NG 12 5GG
P. G. Nicholson, Department of Geology and Applied Geology, The University of Glasgow, Glasgow, Gi2 8QQ P. F. Rawson, Department of Geological Sciences, University College London, Gower Street, London, WC1E 6BT A. W. A. Rushton*, British Geological Survey, Nicker Hill, Keyworth,
R. Cave, University College of Wales, Aberystwyth, SY23 3DB
Nottingham, NGI2 5GG
J. C. W. Cope, Department of Geology, University of Wales College of Cardiff, PO Box 914, Cardiff, CFI 3YE
C . T . Scrutton, Department of Geological Sciences, The University of Durham, South Road, Durham, DH! 3LE
D. W. Cripps, formerly Department of Geology, University of Aston (now The Robertson Group, Ty'n-y-Coed, Llanrhos, Llandudno, LL30 1SA)
G . D . Sevastopulo, Department of Geology, Trinity College, Dublin 2, Ireland
D. T. Donovan, Department of Geological Sciences, University College London, Gower Street, London, WC1E 6BT
D. B. Smith, Geoperm, 79 Kenton Road, Newcastle upon Tyne, NE3 4NL A. Straw, Department of Geography, The University, Exeter, EX4 4QE
R. A. Fortey, Department of Palaeontology, British Museum (Natural History), Cromwell Road, London, SW7 5BD
A. R. H. Swan, School of Geological Sciences, Kingston Polytechnic, Penrhyn Road, Kingston upon Thames, KTI 2EE
W. Gibbons, Department of Geology, University of Wales College of Cardiff, PO. Box 914, Cardiff, CF1 3YE
J. C. M. Taylor, 41 High Trees Road, Reigate, RH2 7EN
C. C. Graham*, British Geological Survey, Murchison House, West Mains Road, Edinburgh, EH9 3LA
G. Warrington*, British Geological Survey, Nicker Hill, Keyworth, Nottingham, NGI2 5GG
P. D. Guion, Department of Geology and Physical Science, Oxford Polytechnic, Headington, Oxford, OX3 0BP
I.M. West, Department of Geology, The University, Southampton, SO9 5NH
J. M. Hancock, Department of Geology, Royal School of Mines, Imperial College, Prince Consort Road, London, SW7 2BP
W. A. Wimbledon, 15 Stoney Lane, Shaw, Newbury, RG13 2NG
C. H. Holland, Department of Geology, Trinity College, Dublin 2, Ireland M. K. Howarth, Department of Palaeontology, British Museum (Natural History), Cromwell Road, London, SW7 5BD
*These contributors publish with the approval of the Director, British Geological Survey (NERC).
Editorial To produce a compilative work such as this atlas, which has necessarily involved a large number of contributors, inevitably generates problems; to pretend there have been no problems in its gestation would be disingenuous. The selection of specialist contributors to each System, and to each part of each System seemed, at the time, the best way to proceed; inevitably this led to problems. Firstly there were those who produced high quality work promptly, whose understandable impatience has had to be reconciled with those whose more leisurely approach could not be hastened. Then there were a few who produced work which was not to our required specifications and for them, and those who failed to produce anything, replacement contributors had to be found. Some replacement contributors also failed to produce the required work and the editors have perforce become major contributors to some of the sections of this atlas in areas outside of their own specialist knowledge. The editors have also had to reconcile some strongly conflicting interpretations of contiguous maps from different authors, and would like to acknowledge the willingness of these authors to allow their views to be compromised to produce a coherent series of maps. The final result is the editors' responsibility. In total there are over 30 contributors to whom the editors have to express their thanks, one of whom, Adrian Rushton, was the only one of the original co-ordinators to stay the course. In addition there are numerous others whose assistance is acknowledged in each chapter. Finally, there are the large number of individuals whose contributions may be too minor for formal acknowledgement, but whose unstinted provision of odd snippets of information has saved countless hours of work by editors and contributors and has made the maps more accurate or up to date. Our colleagues on the Society's Stratigraphy Committee, under whose auspices the Atlas project evolved, have provided a constant spur to our activities and have provided innumerable suggestions to improve the publication. The Atlas has been produced under the co-ordinating editorship of John Cope, who has had special responsibilities for the Precambrian to Permian maps and text; Peter Rawson's responsibility has involved the editing of the
Technical Note The coloured maps in this Arias were produced on Apple Macintosh computers, principally a IICX 4/40. The software used was Aldus Freehand. Final film for the printer was produced on a Linotronic 300R Image Setter.
Triassic to Recent contributions, whilst Keith Ingham, as editor in charge of all artwork, has been responsibile for choices of colour and drafting schemes; the final appearance of the Atlas owes much to his skills. The Geological Society has provided encouragement and major financial support for the project since its inception. The Society acknowledges the generous support of Industry in the production of this Atlas, in particular from the following companies: Shell UK Exploration and Production Mobil North Sea Limited BP Exploration Company Limited LASMO plc British Gas plc The RTZ Corporation plc Getty Oil Sun International Exploration and Production Company Limited Marathon Oil UK Limited Hamilton Brothers Oil and Gas Limited Total Oil Marine plc ARCO British Limited In order to make the Atlas available at a price which should enable widespread circulation, substantial contributions were made from the Henry Wood Trust Fund of the Geological Society and also from the Royal Society of London. JCWC The editors are indebted to Professor Bernard Leake for his unstinted support and encouragement in seeing this Atlas into production. We have benefited from the experience of staff at Lovell Johns and of Alan Cooper, who have provided considerable advice in the drafting of the maps.
Editorial to the revised reprint This edition of the atlas is a reduced-size reprint of the original A3 atlas. That work, the most expensive publishing venture hitherto undertaken by the Society, has covered its costs, so the opportunity exists to produce this smaller sized version at a low cost. Inevitably in the time that has elapsed since the atlas was produced there have been some major publications that have impinged upon the map reconstructions. The decision was made, to enable the price to be kept as low as possible, that no amendments would be made to the maps or text for this edition, but that this editorial page could be used to draw the readers' attention to points where revision is needed. It is not possible in the limited space available to mention the large number of minor modifications needed to the maps, but an attempt has been made here to summarize some of the more significant changes required.
Terranes. Controversy still exists about some of the terrane boundaries shown on the map, but the hypothesis of Terrane Tectonics has gained wide acceptance. Unresolved problems include the status of the Grampian Group - was it, as the map suggests, the basal part of the Dalradian Supergroup (in which case the Great Glen Fault is a terrane boundary) or was it the uppermost group of the Moine Supergroup (in which case the Great Glen Fault is simply a post-Moine fault)? At the southern end of the Dalradian outcrop similar problems concern the fossiliferous rocks that crop out along the Highland Boundary Fault zone across the Scottish Highlands and westwards to western Ireland. These fossiliferous units may be part of a largely hidden terrane (the Highland Border Subterrane as depicted on the map), whilst others argue that they are the youngest component of the Dalradian Supergroup. For a detailed account of the Scottish Precambrian rocks, the reader is referred to the revised Precambrian correlation charts (Gibbons & Harris 1994). Precambrian. New dates for the Torridonian (Turnbull et al. 1996) suggest a Stoer Group age of 11994-70 Ma (P~la) and 977+39 Ma for the Applecross Formation (PC Ib). New dates for the Precambrian-Cambrian boundary (Bowring et al. 1993; Landing et al. 1998) suggest that map PC3a should be around 565 Ma and PC3b around 545 Ma. Cambrian. New dates for the Cambrian (Bowring et al. 1993; Davidek et al. 1998; Landing et al. 1998) require revisions to the dates of the Cambrian maps. Revised suggested dates are: C l a 520 Ma, C l b 510 Ma, g 2 a 498 Ma, ~2b 520 Ma. Ordovician. Revisions of the British Ordovician series (Fortey et al. 1995) propose that the Llandeilo Series be largely subsumed within an enlarged Llanvirn Series and relegated to stage-level, with the gracilis Biozone marking the base of a revised Caradoc Series. New dates for the Ordovician (Tucker & McKerrow 1995; Davidek et al. 1998) also require changes as follows: Ola 487 Ma, Olb 478 Ma (now = earliest Whiterock), O2a 466 Ma, O2b 458 Ma, O3a 454 Ma, O3b 446 Ma, O4a 487 Ma, O4b 480 Ma, O5a 466 Ma, O5b 458 Ma (under the new scheme for the British Ordovician series, this would now be earliest Caradoc), O6a 456 Ma, O6b 455 Ma, O7a 454 Ma, O7b 451 Ma, O8a 446 Ma, O8b 444 Ma. Silurian. New dates for the Silurian (Tucker & McKerrow 1995) suggest the following revised dates for the Silurian maps: Sla 443 Ma, Slb 434 Ma, S2a 433 Ma, S2b 431 Ma, S3a 427 Ma, S3b 424 Ma, S4a 423 Ma, S4b 422 Ma, S5a 421 Ma, S5b 420 Ma, S6a 443 Ma, S6b 434 Ma, $7 431 Ma, S8a 427 Ma, S8b 419 Ma, $9 418 Ma. The record of rapid Rhuddanian and Aeronian subsidence in the Woolhope and Usk basins recorded by Butler et al. (1997) requires modification to the first Silurian map (Sla). These basins, that developed during rifting of the western part of the Midland Platform, were bordered north-westwards by the Neath Disturbance and eastwards by the Malvern line. Huntley Quarry (indicated on the map) lies at the eastern boundary of this area where thinner marginal successions accumulated. The southern margin of this depositional area (as for map Slb) is conjectural. For northern parts of southern Britain, it is now clear that Eastern Avalonia was closer to Laurentia by the late Ordovician than was envisaged when the Atlas was drawn up and that its initial docking against the Laurentian margin took place earlier in the Silurian. It was completed in early Devonian times, with a component of anticlockwise rotation (Soper & Woodcock 1990). By mid-Ludlow times a migrating foreland basin was established over the Lake District that accumulated some 7 km of turbidites, probably largely derived from a Southern Uplands source, but with input from Scandia too (Cooper et al. 1993) (requiring amendments to maps S5a and S5b). By the start of Pfidoli times this basin was largely filled and was subtidal (map S8b). Devonian. New dates are suggested for three of the Devonian maps. The suggested dates are: D1 415 Ma, D2 400 Ma, D3 375 Ma, D4 368 363 Ma. Cooper et al. (1993) suggested that the post-Acadian Mell Fell Conglomerate of the Shap area could be of Middle Devonian age. Within the palaeogeographical picture presented in the Atlas, this deposit would fit more easily onto map D4 rather than D3.
Carboniferous. Dating of the Carboniferous continues to present problems and there are major unresolved discrepancies between the dates obtained by
the Ar-Ar method and those obtained from Pb/U, Rb/Sr and K/Ar methods (Menning et al. 1997). The suggested compromise dates for the maps are: C1 354 Ma, C2 348 Ma, C3 343 Ma, C4 337 Ma, C5 325 Ma, C6 315 Ma, C7 312 Ma, C8 309 Ma, C9 304 Ma. The identification of early Dinantian evaporites in the Solway Basin extending over more than 1000 km 2 implies arid and semi-arid conditions across that basin around Chadian and Arundian ages (Ward 1998). Cyclic successions were correlated with glacio-eustatic sea-level oscillations and the identification of this facies requires evaporite symbols over the Solway Basin on maps C3 and C4.
Permian and Triassic. No changes are proposed for the maps of these periods. Edwards et al. (1997) recognized that the Crediton Trough of central Devon is an E-W basin that has a fill of Late Carboniferous-Early Permian sediments that is overlain unconformably by a Late Permian sequence, the two successions being separated by a hiatus of at least 20 million years duration. Jurassic. For the Jurassic, the late Toarcian uplift has been now firmly associated with thermal doming of the North Sea centred on the Rattray volcanic centre. The effects of this thermal doming have been discussed in detail by Underhill & Partington (1993); they show that the area affected was greater than 1250 km in diameter and have charted the history of the doming and the subsequent deflation that persisted through to the late Oxfordian. This work allows better palaeogeographical restraint on the maps for the Aalenian-Oxfordian interval and also suggests that there was a marine connection from the Hebrides through to the Moray Firth across the Great Glen in Bathonian-Oxfordian times (Underhill & Partington 1993, fig. 4). Cretaceous. In the Upper Cretaceous the palaeogeographies represented by maps K4a and K4b may now be considered as somewhat conservative reconstructions. The identification of doming centred on the Irish Sea (Cope 1994) initially suggested from inversion indicated by Apatite Fission Track Analysis (Lewis et al. 1992, Holliday 1993) strongly suggests that there was a considerable Chalk thickness not only over the Irish Sea, but right over mainland Wales (at least by the Campanian, if not much earlier). The amount of land shown over the Lake District and the Southern Uplands may also be too great. Cope (1998) has reconstructed a likely Jurassic and Cretaceous cover for the Irish Sea area and its environs. Palaeogene. The doming of the Irish Sea, referred to under 'Cretaceous' above probably took place in the earliest Palaeocene and map Pgl should be modified to show an Irish Sea centred uplift of some 2-2.5 km whose effects reached at least as far south as the Chiltern Hills and are responsible for the 2~ regional south-easterly dip of the Anglo-Welsh area (Cope 1994). Neogene and Quaternary. No changes are proposed for these maps. References BOWRING, S. A., GROTZINGER, J. P., ISACHSEN, C. E., KNOLL, A. H., PELECHATY, S. M. & KOLOSOV, P. 1993. Calibrating rates of Early Cambrian evolution. Science, 261, 1293 1298. BOILER, A. J., WOODCOCK,N. H. & STEWARX,D. M. 1997. The Woolhope and Usk Basins: Silurian rift basins revealed by subsurface mapping of the southern Welsh Borderland. Journal of the Geological Society, London, 154, 209-223. COOPER, A. M., MILLWARD,D., JOHNSON, E. W. • SOPER, N. J. 1993. The early Palaeozoic evolution of northwest England. Geological Magazine, 130, 711-724. COPE, J. C. W. 1994. A latest Cretaceous hotspot and the south-easterly tilt of Britain. Journal of the Geological Society, London, 151, 905-908. -1998. The Mesozoic and Tertiary history of the Irish Sea. In: MEADOWS, N. S., TRUEBLOOD,S., HARDMAN,N. & COWAN, G. (eds). The Petroleum Geology of the Irish Sea and adjacent areas. Geological Society, London, Special Publications, 124, 48-59. [dated 1997] DAVIDEK, K., LANDING, E., BOWRING, S. A., WESTROP, S. R., RUSHTON, A. W. A. FORTEY, R. A. & ADRAIN,J. 1998. New uppermost Cambrian U-Pb dates from Avalonian Wales and the age of the CambrianOrdovician boundary. Geological Magazine, 135, 303-309. EDWARDS, A., WARRINGTON,G., SCRIVENER,R. C., JONES, N. S., HASLAM, H. W. & AULT, A. 1997. The Exeter Group, south Devon, England: a contribution to the early post-Variscan stratigraphy of northwest Europe. Geological Magazine, 134, 177-197. FORTEY, R. A., HARPER,D. A. T., INGHAM,J. K., OWEN, A. W. & RUSHTON, A. W. A. 1995. A revision of Ordovician series and stages from the historical type area. Geological Magazine, 132, 15-30. GIBBONS, W. & HARRIS, A. L. (eds). 1994. A Revised Correlation of Preeambrian Rocks in the British Isles. Geological Society, London, Special Report, 22. HOLLIDAY,D. W. 1993. Mesozoic cover over northern England: interpretation of fission track data. Journal o f the Geological Society, London, 150, 657 660. LANDING,E., BOWRING,S. A., DAVIDEK,K. L., WESTROP, S. R., GEYER, G. & HELDMAIER,W. 1998. Duration of the early Cambrian: U-Pb ages of volcanic ashes from Avalon and Gondwana. Canadian Journal of Earth Sciences, 35, 329-338.
LEWIS, C. L. E., GREEN, P. F., CARTER,A. & HURFORD, A. J. 1992. Elevated K/T palaeotemperatures throughout Northwest England; three kilometres of Tertiary erosion? Earth and Planetary Science Letters, 112, 131-145. MENNING, M., WEYER, D., DROZDZEWSKI,G. • VANAMEROM,H. W. J. 1997. Carboniferous Time Scales Revised 1997 Time scale A (rain. ages) and Time Scale B (max. ages) Use of geological time indicators. Newsletter on Carboniferous Stratigraphy, 15. International Union of Geological Sciences. SOPER, N. J. t~ WOODCOCK, N. H. 1990. Silurian collision and sediment dispersal patterns in southern Britain. Geological Magazine, 127, 527542. TUCKER, R. D. & MCKERROW, W. S. 1995. Early Paleozoic geochronology: A review in light of new U-Pb zircon ages from Newfoundland and Britain. Canadian Journal of Earth Sciences, 32, 368-379. TURNBULL, M. J. M., WHITEHOUSE, M. J. & MOORBATH, S. 1996. New isotopic age determinations for the Torridonian, NW Scotland. Journal of the Geological Society, London, 153, 955-964. UNDERHILL,J. R. t~ PARTtN•TON, M. A. 1993. Jurassic thermal doming and deflation in the North Sea: implications of the sequence stratigraphic evidence. In: PARKER,J. R. (ed.) Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. Geological Society, London, 337-345. WARD, J. 1998 Early Dinantian evaporites of the Easton-1 well, Solway Basin, onshore, Cumbria, England. In: MEADOWS,N. S., TRUEBLOOD,S., HARDMAN, N. & COWAN, G. (eds). The Petroleum Geology of the Irish Sea and Adjacent Areas. Geological Society, London, Special Publications, 124, 277-296. [dated 1997] JCWC
Contents vii ix x xi xiii
List of Contributors Editorial Editorial to the revised edition Introduction Key
Terranes
1
T 1: Terranes
3
Precambrian
5 7 7
C3: C4: C5: C6:
C7: C8: C9:
Latest Courceyan-early Chadian Arundian Brigantian Namurian: Arnsbergian Namurian: Marsdenian Westphalian A (Langsettian) Westphalian D
Permian PCla: PClb: P4~2:
Upper Proterozoic Stoer Group Upper Proterozoic Torridon Group Northwestern British Isles: Upper Proterozoic Dalradian Supergroup PC3a,b: Southern British Isles: late Precambrian
Cambrian Cla: Clb: C2a: 4~2b:
Southern Southern Southern Northern
British British British British
Isles: Isles: Isles: Isles:
Comley St David's Merioneth Comley
Ordovician Ola: Olb: O2a: O2b: O3a: O3b: O4a: O4b: O5a: O5b: O6a: O6b: O7a: O7b: O8a: O8b:
Northern British Northern British Northern British Northern British Northern British Northern British Southern British Southern British Southern British Southern British Southern British Southern British Southern British Woolstonian) Southern British Southern British Southern British
Isles: Tremadoc Isles: mid Arenig Isles: late Llanvirn Isles: mid Llandeilo Isles: mid Caradoc Isles: mid Ashgill Isles: Tremadoc Isles: Arenig Isles: late Llanvirn Isles: mid Llandeilo Isles: Caradoc (Costonian) Isles: Caradoc (Harnagian) Isles: Caradoc (Longvillian including Isles: Caradoc (Onnian) Isles: mid Ashgill (Rawtheyan) Isles: late Ashgill (Hirnantian)
Silurian Sla: Slb: S2a: S2b: S3a: S3b: S4a: S4b: S5a: S5b: S6a: S6b: $7: S8a: S8b: $9:
Southern British Isles: latest Ashgill-early Llandovery Southern British Isles: Llandovery (Aeronian) Southern British Isles: Llandovery (Aeronian) Southern British Isles: (Telychian) Southern British Isles: Wenlock (Sheinwoodian) Southern British Isles: Wenlock (Homerian) Southern British Isles: Ludlow (mid Gorstian) Southern British Isles: Ludlow (late Gorstian) Southern British Isles: (early Ludfordian) Southern British Isles: (late Ludfordian) Northern British Isles: Llandovery (Rhuddanian) Northern British Isles: Llandovery (Aeronian) Northern British Isles: Llandovery (Telychian) Northern British Isles: Wenlock (Sheinwoodian) Southern British Isles: earliest Pfidoli All British Isles: mid Pfidoll
9 11 13 15 15 17 17 19 21 21 23 23 25 25 27 27 29 29 31 31 33 33 35 35 37 39 39 41 41 43 43 45 45 47 47 49 49 51 53 53 55
Devonian
57
D 1: D2: D3: D4:
59 61 63 65
Lochkovian Pragian-Emsian boundary Givetian Frasnian-early Famennian
P 1: P2a: P2b: P3a: P3b: P4a: P4b: P4c: P4d:
87 Early Permian Lower part of English Zechstein Cycle 1 and equivalents Upper part of English Zechstein Cycle 1 and equivalents Lower part of English Zechstein Cycle 2 and equivalents Upper part of English Zechstein Cycle 2 Lower part of English Zechstein Cycle 3 and equivalents Upper part of English Zechstein Cycle 3 and equivalents Upper middle part of English Zechstein Cycle 4 and equivalents English Zechstein Cycle 5 and equivalents
Triassic Trla: Trlb: Tr2: Tr3a: Tr3b: Tr4a: Tr4b: Tr4c:
J10: Jl la-d:
Late Ryazanian (Berriasian) Mid Hauterivian Late Aptian Latest Albian Early Cenomanian Late Campanian
Palaeogene and Neogene Pg 1: Pg2a: Pg2b: Pg2c: Pg2d: Pg3: Ng 1:
Palaeocene Early Palaeocene and the sub-Palaeocene floor Early Eocene Mid Eocene Late Eocene Mid-late Oligocene Miocene-Pliocene
Quaternary Carboniferous CI: Devonian-Carboniferous boundary (latest FamennianC2:
earliest Courceyan) Mid Courceyan
67 69 71
Qla: QI b: Q2:
95 95
99 99 101 103 103 105 105 105
107 Early Hettangian Early Pliensbachian (Carixian) Late Pliensbachian (Domerian) Mid Toarcian Early Aalenian Late Aalenian Early Bajocian Late Bajocian Mid Bathonian Late Bathonian Early Callovian Mid Callovian Mid Oxfordian Late Kimmeridgian (early Volgian) Portlandian
Cretaceous K 1: K2a: K2b: K3: K4a: K4b:
89 91 91 93 93 95 95
97 Early to mid Sythian Earliest Anisian to earliest Carnian Earliest Anisian to earliest Carnian Earliest Anisian to earliest Carnian Latest Norian to earliest Rhaetian Mid to late Rhaetian Late Carnian (detail) Late Rhaetian (detail)
Jurassic J 1: J2a: J2b: J3: J4a: J4b: J5: J6a: J6b: J7a: J7b: J8: J9:
73 75 77 79 81 83 85
109 111 111 113 115 115 117 119 119 121 121 123 125 127 129 131 133 135 135 137 139 139 141 143 145 145 145 145 147 147
149
Quaternary geography 151 Quaternary crustal movement 151 Present-day seabed sediment distribution and topography 153
Editorial to the revised reprint This edition of the atlas is a reduced-size reprint of the original A3 atlas. That work, the most expensive publishing venture hitherto undertaken by the Society, has covered its costs, so the opportunity exists to produce this smaller sized version at a low cost. Inevitably in the time that has elapsed since the atlas was produced there have been some major publications that have impinged upon the map reconstructions. The decision was made, to enable the price to be kept as low as possible, that no amendments would be made to the maps or text for this edition, but that this editorial page could be used to draw the readers' attention to points where revision is needed. It is not possible in the limited space available to mention the large number of minor modifications needed to the maps, but an attempt has been made here to summarize some of the more significant changes required.
Terranes. Controversy still exists about some of the terrane boundaries shown on the map, but the hypothesis of Terrane Tectonics has gained wide acceptance. Unresolved problems include the status of the Grampian Group - was it, as the map suggests, the basal part of the Dalradian Supergroup (in which case the Great Glen Fault is a terrane boundary) or was it the uppermost group of the Moine Supergroup (in which case the Great Glen Fault is simply a post-Moine fault)? At the southern end of the Dalradian outcrop similar problems concern the fossiliferous rocks that crop out along the Highland Boundary Fault zone across the Scottish Highlands and westwards to western Ireland. These fossiliferous units may be part of a largely hidden terrane (the Highland Border Subterrane as depicted on the map), whilst others argue that they are the youngest component of the Dalradian Supergroup. For a detailed account of the Scottish Precambrian rocks, the reader is referred to the revised Precambrian correlation charts (Gibbons & Harris 1994). Precambrian. New dates for the Torridonian (Turnbull et al. 1996) suggest a Stoer Group age of 11994-70 Ma (P~la) and 977+39 Ma for the Applecross Formation (PC Ib). New dates for the Precambrian-Cambrian boundary (Bowring et al. 1993; Landing et al. 1998) suggest that map PC3a should be around 565 Ma and PC3b around 545 Ma. Cambrian. New dates for the Cambrian (Bowring et al. 1993; Davidek et al. 1998; Landing et al. 1998) require revisions to the dates of the Cambrian maps. Revised suggested dates are: C l a 520 Ma, C l b 510 Ma, g 2 a 498 Ma, ~2b 520 Ma. Ordovician. Revisions of the British Ordovician series (Fortey et al. 1995) propose that the Llandeilo Series be largely subsumed within an enlarged Llanvirn Series and relegated to stage-level, with the gracilis Biozone marking the base of a revised Caradoc Series. New dates for the Ordovician (Tucker & McKerrow 1995; Davidek et al. 1998) also require changes as follows: Ola 487 Ma, Olb 478 Ma (now = earliest Whiterock), O2a 466 Ma, O2b 458 Ma, O3a 454 Ma, O3b 446 Ma, O4a 487 Ma, O4b 480 Ma, O5a 466 Ma, O5b 458 Ma (under the new scheme for the British Ordovician series, this would now be earliest Caradoc), O6a 456 Ma, O6b 455 Ma, O7a 454 Ma, O7b 451 Ma, O8a 446 Ma, O8b 444 Ma. Silurian. New dates for the Silurian (Tucker & McKerrow 1995) suggest the following revised dates for the Silurian maps: Sla 443 Ma, Slb 434 Ma, S2a 433 Ma, S2b 431 Ma, S3a 427 Ma, S3b 424 Ma, S4a 423 Ma, S4b 422 Ma, S5a 421 Ma, S5b 420 Ma, S6a 443 Ma, S6b 434 Ma, $7 431 Ma, S8a 427 Ma, S8b 419 Ma, $9 418 Ma. The record of rapid Rhuddanian and Aeronian subsidence in the Woolhope and Usk basins recorded by Butler et al. (1997) requires modification to the first Silurian map (Sla). These basins, that developed during rifting of the western part of the Midland Platform, were bordered north-westwards by the Neath Disturbance and eastwards by the Malvern line. Huntley Quarry (indicated on the map) lies at the eastern boundary of this area where thinner marginal successions accumulated. The southern margin of this depositional area (as for map Slb) is conjectural. For northern parts of southern Britain, it is now clear that Eastern Avalonia was closer to Laurentia by the late Ordovician than was envisaged when the Atlas was drawn up and that its initial docking against the Laurentian margin took place earlier in the Silurian. It was completed in early Devonian times, with a component of anticlockwise rotation (Soper & Woodcock 1990). By mid-Ludlow times a migrating foreland basin was established over the Lake District that accumulated some 7 km of turbidites, probably largely derived from a Southern Uplands source, but with input from Scandia too (Cooper et al. 1993) (requiring amendments to maps S5a and S5b). By the start of Pfidoli times this basin was largely filled and was subtidal (map S8b). Devonian. New dates are suggested for three of the Devonian maps. The suggested dates are: D1 415 Ma, D2 400 Ma, D3 375 Ma, D4 368 363 Ma. Cooper et al. (1993) suggested that the post-Acadian Mell Fell Conglomerate of the Shap area could be of Middle Devonian age. Within the palaeogeographical picture presented in the Atlas, this deposit would fit more easily onto map D4 rather than D3.
Carboniferous. Dating of the Carboniferous continues to present problems and there are major unresolved discrepancies between the dates obtained by
the Ar-Ar method and those obtained from Pb/U, Rb/Sr and K/Ar methods (Menning et al. 1997). The suggested compromise dates for the maps are: C1 354 Ma, C2 348 Ma, C3 343 Ma, C4 337 Ma, C5 325 Ma, C6 315 Ma, C7 312 Ma, C8 309 Ma, C9 304 Ma. The identification of early Dinantian evaporites in the Solway Basin extending over more than 1000 km 2 implies arid and semi-arid conditions across that basin around Chadian and Arundian ages (Ward 1998). Cyclic successions were correlated with glacio-eustatic sea-level oscillations and the identification of this facies requires evaporite symbols over the Solway Basin on maps C3 and C4.
Permian and Triassic. No changes are proposed for the maps of these periods. Edwards et al. (1997) recognized that the Crediton Trough of central Devon is an E-W basin that has a fill of Late Carboniferous-Early Permian sediments that is overlain unconformably by a Late Permian sequence, the two successions being separated by a hiatus of at least 20 million years duration. Jurassic. For the Jurassic, the late Toarcian uplift has been now firmly associated with thermal doming of the North Sea centred on the Rattray volcanic centre. The effects of this thermal doming have been discussed in detail by Underhill & Partington (1993); they show that the area affected was greater than 1250 km in diameter and have charted the history of the doming and the subsequent deflation that persisted through to the late Oxfordian. This work allows better palaeogeographical restraint on the maps for the Aalenian-Oxfordian interval and also suggests that there was a marine connection from the Hebrides through to the Moray Firth across the Great Glen in Bathonian-Oxfordian times (Underhill & Partington 1993, fig. 4). Cretaceous. In the Upper Cretaceous the palaeogeographies represented by maps K4a and K4b may now be considered as somewhat conservative reconstructions. The identification of doming centred on the Irish Sea (Cope 1994) initially suggested from inversion indicated by Apatite Fission Track Analysis (Lewis et al. 1992, Holliday 1993) strongly suggests that there was a considerable Chalk thickness not only over the Irish Sea, but right over mainland Wales (at least by the Campanian, if not much earlier). The amount of land shown over the Lake District and the Southern Uplands may also be too great. Cope (1998) has reconstructed a likely Jurassic and Cretaceous cover for the Irish Sea area and its environs. Palaeogene. The doming of the Irish Sea, referred to under 'Cretaceous' above probably took place in the earliest Palaeocene and map Pgl should be modified to show an Irish Sea centred uplift of some 2-2.5 km whose effects reached at least as far south as the Chiltern Hills and are responsible for the 2~ regional south-easterly dip of the Anglo-Welsh area (Cope 1994). Neogene and Quaternary. No changes are proposed for these maps. References BOWRING, S. A., GROTZINGER, J. P., ISACHSEN, C. E., KNOLL, A. H., PELECHATY, S. M. & KOLOSOV, P. 1993. Calibrating rates of Early Cambrian evolution. Science, 261, 1293 1298. BOILER, A. J., WOODCOCK,N. H. & STEWARX,D. M. 1997. The Woolhope and Usk Basins: Silurian rift basins revealed by subsurface mapping of the southern Welsh Borderland. Journal of the Geological Society, London, 154, 209-223. COOPER, A. M., MILLWARD,D., JOHNSON, E. W. • SOPER, N. J. 1993. The early Palaeozoic evolution of northwest England. Geological Magazine, 130, 711-724. COPE, J. C. W. 1994. A latest Cretaceous hotspot and the south-easterly tilt of Britain. Journal of the Geological Society, London, 151, 905-908. -1998. The Mesozoic and Tertiary history of the Irish Sea. In: MEADOWS, N. S., TRUEBLOOD,S., HARDMAN,N. & COWAN, G. (eds). The Petroleum Geology of the Irish Sea and adjacent areas. Geological Society, London, Special Publications, 124, 48-59. [dated 1997] DAVIDEK, K., LANDING, E., BOWRING, S. A., WESTROP, S. R., RUSHTON, A. W. A. FORTEY, R. A. & ADRAIN,J. 1998. New uppermost Cambrian U-Pb dates from Avalonian Wales and the age of the CambrianOrdovician boundary. Geological Magazine, 135, 303-309. EDWARDS, A., WARRINGTON,G., SCRIVENER,R. C., JONES, N. S., HASLAM, H. W. & AULT, A. 1997. The Exeter Group, south Devon, England: a contribution to the early post-Variscan stratigraphy of northwest Europe. Geological Magazine, 134, 177-197. FORTEY, R. A., HARPER,D. A. T., INGHAM,J. K., OWEN, A. W. & RUSHTON, A. W. A. 1995. A revision of Ordovician series and stages from the historical type area. Geological Magazine, 132, 15-30. GIBBONS, W. & HARRIS, A. L. (eds). 1994. A Revised Correlation of Preeambrian Rocks in the British Isles. Geological Society, London, Special Report, 22. HOLLIDAY,D. W. 1993. Mesozoic cover over northern England: interpretation of fission track data. Journal o f the Geological Society, London, 150, 657 660. LANDING,E., BOWRING,S. A., DAVIDEK,K. L., WESTROP, S. R., GEYER, G. & HELDMAIER,W. 1998. Duration of the early Cambrian: U-Pb ages of volcanic ashes from Avalon and Gondwana. Canadian Journal of Earth Sciences, 35, 329-338.
LEWIS, C. L. E., GREEN, P. F., CARTER,A. & HURFORD, A. J. 1992. Elevated K/T palaeotemperatures throughout Northwest England; three kilometres of Tertiary erosion? Earth and Planetary Science Letters, 112, 131-145. MENNING, M., WEYER, D., DROZDZEWSKI,G. • VANAMEROM,H. W. J. 1997. Carboniferous Time Scales Revised 1997 Time scale A (rain. ages) and Time Scale B (max. ages) Use of geological time indicators. Newsletter on Carboniferous Stratigraphy, 15. International Union of Geological Sciences. SOPER, N. J. t~ WOODCOCK, N. H. 1990. Silurian collision and sediment dispersal patterns in southern Britain. Geological Magazine, 127, 527542. TUCKER, R. D. & MCKERROW, W. S. 1995. Early Paleozoic geochronology: A review in light of new U-Pb zircon ages from Newfoundland and Britain. Canadian Journal of Earth Sciences, 32, 368-379. TURNBULL, M. J. M., WHITEHOUSE, M. J. & MOORBATH, S. 1996. New isotopic age determinations for the Torridonian, NW Scotland. Journal of the Geological Society, London, 153, 955-964. UNDERHILL,J. R. t~ PARTtN•TON, M. A. 1993. Jurassic thermal doming and deflation in the North Sea: implications of the sequence stratigraphic evidence. In: PARKER,J. R. (ed.) Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. Geological Society, London, 337-345. WARD, J. 1998 Early Dinantian evaporites of the Easton-1 well, Solway Basin, onshore, Cumbria, England. In: MEADOWS,N. S., TRUEBLOOD,S., HARDMAN, N. & COWAN, G. (eds). The Petroleum Geology of the Irish Sea and Adjacent Areas. Geological Society, London, Special Publications, 124, 277-296. [dated 1997] JCWC
Introduction Almost forty years have elapsed since Leonard Wills published his 'Palaeogeographical Atlas of the British Isles'; those forty years have seen a revolution in the earth sciences which has overturned many earlier ideas in geology. Palaeogeography has been affected just as much as other parts of the subject by this change. The advent of the plate tectonic theory has transformed our ideas of the Lower Palaeozoic palaeogeographical evolution of the British area and has rendered totally obsolete many aspects of Wills' maps. The application of plate tectonics has caused us to produce totally new palaeogeographical models for the late Precambrian and early Palaeozoic, and has emphasised that definitive palaeogeographies for this time interval cannot yet be compiled. Wills was at pains to point out that his Atlas was an 'Aunt Sally' at which to 'hurl one's own and other field observations'--our atlas too must be viewed in that light. The Precambrian and Lower Palaeozoic maps are separated for the northern and southern British Isles areas, because for much of that time the two areas were separated by the Iapetus Ocean. The recognition of important terrane boundaries in Scotland and Ireland has led to our attempt to reconstruct a map from the collage of terranes now in juxtaposition. These early maps owe much to Keith Ingham's experience of the Lower Palaeozoic--Keith would insist here that the maps are very much in the 'Aunt :: Sally' mould, and that his interpretations may not be able to stand the test of time--but they represent a first attempt to piece together the evolution of a particularly fragmented and complex area of the earth's crust. The mid P~idoli Series map is the first one on which northern and southern Britain are in juxtaposition. We realise, however, that most are now agreed that the Iapetus Ocean had essentially disappeared by Wenlock times and the intervening area was probably underlain by continental crust with a residual zone of transpressional convergence between. Having agreed on the palinspastic reconstructions of the maps for the early Palaeozoic, there remained the problem of where to end such reconstructions for the north of Britain. For most terrane boundaries, major movement seems to have ceased by end-Silurian to mid Devonian times, but the Great Glen-Walls Boundary Fault System appears to be the exception. It seems probable that there were significant dextral movements along this fault system, largely in late Devonian and Carboniferous times, but some Mesozoic movement seems probable, and later movement is possible. There is no general agreement on this aspect of displacement history; for the sake of simplicit~r therefore, we have eliminated all palinspastic adjustment in northern Britain from the Devonian/Carboniferous boundary map onwards. This inevitably means that our Devonian reconstructions have an in-built inaccuracy, but this solution seems the best compromise. For the Carboniferous, recent researches have begun to produce a model synthesizing plate tectonics with climatic change, changes in sea-level and patterns of sedimentation. This profound advancement of Carboniferous processes gives us a far better insight into Carboniferous palaeogeographical
changes. The predictive consequences of this understanding have had important economic repercussions. The Carboniferous is now. one of the main objectives in current petroleum exploration in the British Isles and on the surrounding continental shelves, and major reassessment of palaeogeography will no doubt become necessary as more results of the hydrocarbon exploration become available. If the plate tectonic theory has caused us to reconsider totally our ideas of much Palaeozoic palaeogeography, then offshore exploration is the one major factor which has caused a complete re-evaluation of most of our later palaeogeographies. A single borehole can have a devastating effect on a whole suite of palaeogeographical maps. Such was the case with the Mochras Borehole in Gwynedd--which with its extraordinarily thick succession of Lower Jurassic marine sediments, despatched at one blow the Jurassic Welsh Landmass of Wills and countless other workers. The Mochras Borehole was not even offshore, but the release of offshore data consequent upon the offshore exploration programme has produced such a plethora of data that palaeogeographical maps have had to be redrawn completely and (as Peter Rawson has become only too well aware) constantly revised, as more information becomes available. To include some of the latest available information, the editors have not, in all cases, been able to consult with the original authors. The speed with which new data are being released makes it inevitable that some aspects of these maps are already out-dated. Another major factor in the drawing-up of palaeogeographies, particularly when no areas are left blank on the maps, is that the very act of drawing a line on a map, however tentative that line may be, lends it an authenticity which is totally spurious. We claim little for the boundaries on these maps save that, to the best of the knowledge available to us, this is the best place to draw them. Lithofacies symbols on the maps are restricted to areas of outcrop or known subcrop of that lithofacies, or to areas where extrapolation is certain, so that what is fact and what is interpretation should be readily apparent. The names of all the contributors responsible for the production of maps and text are given at the beginning of each System; authors of individual maps or text are identified by initials at the end of each block of text. References in the text are listed at the end of the description for each System. There are several radiometric scales available at the present day. Most of these disagree with each other at one or more points in the scale. A decision was taken early on in the production of the Atlas that the radiometric dates we should use would be those based on the Geological Society compilation (Snelling ed. 1985). We realise that this differs in several respects from other scales currently in use. Finally, we recall the words of Aristotle 'not to seek in any subject greater accuracy than its nature admits'. JCWC
Terranes B. J . B L U C K ,
W. G I B B O N S
The Precambrian and Lower Palaeozoic foundations of the British Isles may be viewed as a series of suspect terranes whose exposed boundaries are pro .minent fault systems of various kinds, each with an unproven amount of displacement. There are indications that they accreted to their present configuration between late Precambrian and Carboniferous times. From north to south they are as follows. In northwest Scotland the Hebridean terrane (Laurentian craton in the foreland of the Caledonian Orogen) comprises an Archaean and Lower Proterozoic gneissose basement (Lewisian) overlain by an undeformed cover of Upper Proterozoic red beds and Cambrian to early mid Ordovician shallow marine sediments. The terrane is cut by the Outer Isles Thrust, a rejuvenated Proterozoic structure, and is bounded to the southeast by the Moine Thrust zone, within the hanging wall of which lies a Proterozoic metamorphic complex (Moine Supergroup) which constitutes the Northern Highlands terrane. The Moine Thrust zone represents an essentially orthogonal closure of perhaps 100 km which took place during OrdovicianSilurian times (Elliott & Johnson 1980). The Northern Highlands terrane records both Precambrian and late Ordovician to Silurian tectonometamorphic events (Dewey & Pankhurst 1970) and linkage with the Hebridean terrane is provided by slices of reworked Lewisian basement within the Moine Supergroup (Watson 1983). To the southwest of the Great Glen-Walls Boundary Fault system lies the Central Highlands (Grampian) terrane, an area dominated by the late Proterozoic Dalradian Supergroup which is underlain by a gneissic complex (Central Highland Granulites) that has been variously interpreted as either older basement to, or a deeper level of, the Dalradian Supergroup. Moine and Central Highlands rocks show several lithological similarities and may represent different parts of the same metamorphic complex but there appear to be differences in their respective metamorphic histories. Dalradian rocks were deformed both prior to 590 Ma (Rogers et al. 1989) and during Early Palaeozoic times. The earlier deformations produced two major nappes which diverge from a steep zone parallel to the Great Glen Fault. Unlike the Moine Supergroup, the later events reached a Late Cambrian to Early Ordovician climax, the Grampian event, producing the main regional metamorphism and leading to the final substantial uplift. As there are no macrofaunas found in these metamorphic terranes, there is no constraint on their provenance, nor is there any clear indication of their crustal setting, although by early Silurian times they are firmly identified with the accretionary margin of Laurentia. Caught within a splay between the Great Glen Fault and the Loch Gruinard Fault lies the south Islay-Colonsay suspect terrane, characterized by a 1.8 Ga gneissose basement and undated lower grade metasedimentary cover (Colonsay Group). The latter shows several similarities to the Dalradian Supergroup, but linkage has not been proven (Fitches & Maltman 1984; Bentley et al. 1988). Also associated with, and perhaps lying within, the Great Glen-Walls Boundary Fault system is the Unst-Fetlar suspect terrane of the east Shetlands. Here, northeastward ophiolite obduction is interpreted as Silurian in age, these movements having post-dated an earlier, probably Ordovician, tectonometamorphism (Cannat 1989; Roddam & Miller 1989). The Great Glen-Walls Boundary Fault belt is a major crustal fracture system believed to involve a substantial pre-Middle Devonian sinistral displacement as well as post-Middle Devonian dextral movements. The juxtaposition of metamorphic terranes with perhaps different deformational histories along this belt suggests that unknown terranes may once have existed along the line. The southern margin of the Central Highlands terrane is defined by the Clew Bay-Highland Boundary Fault belt which involves both strike-slip and northwest dipping reverse elements: it truncates Dalradian metamorphic zones. Immediately south of this line in Scotland lie discontinuous slivers of rocks grouped within the Highland Border Complex and comprising carbonates, black shaley mudstones, basic volcanics, volcaniclastic sediments and various arenites, resting on a dismembered ophiolitic basement, the age of which may be determined by the date of the 537 + II Ma for the Bute amphibolite (Dempster & Bluck 1991). The oldest unit yielding macrofossils in the complex is the black limestone and shale sequence of late Early Cambrian age at Callander: the fauna is Laurentian. Most of the rocks however have been dated from Late Canadian (Arenig) through to probable Caradoc or later with a pre-Caradoc hiatus (Bluck et al. 1984; Ingham et al. 1986) and have most recently been interpreted as representing disrupted slivers of a back-arc basin and associated platform margin of the northwest side of a Midland valley magmatic arc terrane (Bluck et al. 1984). It is significant that there is no proven Dairadian detritus in any of these Highland Border rocks at a time when the Central Highlands terrane was undergoing profound elevation. The complex is overlain unconformably by Lower Old Red Sandstone and there is evidence that it extends subsurface 9 1992The GeologicalSociety
& J . K. I N G H A M
for some distance to the southeast of the Highland Boundary Fault belt. Similar lithologies are known from west Ireland around Clew Bay and these also appear to rest on an older disrupted ophiolitic foundation. Within this subterrane are conglomerates of Llandovery age whose provenance is partly from the Central Highlands terrane. The Tremadoc to Llanvirn largely volcanic and volcaniclastic sequences of the adjacent south Mayo subterrane may also represent part of the same back-arc rrgime. The Tourmakeady limestones found in breccias within the volcaniclastic Shangort Formation are remarkably close in fauna and age to the Arenig carbonates of the Highland Border Complex in Scotland. Most of the Midland Valley is covered by Upper Palaeozoic sediments concealing a basement interpreted as an arc massif to a fore-arc area further south (Bluck 1985). In Ireland, part of the Tyrone Igneous Complex may represent an exposure of this arc. Northwest transgressive Llanvirn to Early Llandovery conglomerates, turbiditic sandstones and mudstones at Girvan in southwest Scotland represent a fault-controlled proximal fore-arc setting. Direct correlatives are found in Ireland immediately to the south of the Tyrone Igneous Complex. At Girvan and further to the northeast, siliciclastic sediments record a regression from marine Llandovery to continental Weniock, accompanied by calc-alkaline volcanicity. The foundation to the proximal fore-arc sequences at Girvan comprises the Ballantrae Volcanic Complex, composed of ophiolitic rocks structurally dismembered by strike-slip faults but generated and obducted onto Arenig olistostromes during Early Ordovician times (Bluck 1985). The complex had accreted to the Midland Valley terrane by Llanvirn times and was then undergoing erosion prior to the late Llanvirn transgression. Similar ophiolitic rocks are known from the Tyrone Igneous Complex in Ireland (Hutton et al. 1985). The Connemara suspect terrane in western Ireland is now believed to represent a susbstantially displaced Dalradian block deriving from the Central Highlands terrane, having been relocated sinistrally along the Doon Rock and associated faults. A northwards Llandovery transgression crosses from the Connemara block onto the folded Tremadoc to Llanvirn sequences of the south Mayo subterrane with little displacement and this places a fairly tight constraint on the movements (Bluck & Leake 1986). It may be that the primary docking of the Midland Valley terrane was achieved at about the same time but the constraints here are less well defined. Further strike-slip and reverse movements took place along the Clew Bay-Highland Boundary line as late as Mid Devonian. As the Connemara block was being translated eastwards it was also being thrust southwards over the rhyolitic ramp of the Delaney Dome (Tanner et al. 1989) and the latter may represent a similarly relocated fragment of an originally westward extension of the axial portion of the Midland Valley terrane. The Connemara block is bounded on its south side by the Skirds Rock Fault which brings it into juxtaposition with the Arenig basic volcanic and volcanogenic sediments of the South Connemara Group. This junction is stitched by the Devonian Galway Granite. It has been strongly argued that the Skirds Rock Fault represents an eastwards extension of the Southern Uplands Fault and that the South Connemara Group equates with Early Ordovician sequences in the northern belt of the Southern Uplands (Leake 1963; Williams et al. 1988). It is just feasible, however, that the Skirds Rock Fault is primarily a displaced segment of the Clew Bay line and that the South Connemara Group represents part of a back-arc basinal sequence such as is found in parts of the south Mayo or Clew Bay subterranes. The Southern Uplands are bounded to the north by the Southern Uplands Fault and to the south by the Solway (Iapetus) Line. The recognition of the Orlock Bridge-Kingledores Fault as a major discontinuity suggests that at least two suspect terranes or subterranes exist within the Southern Uplands, one, to the northwest, dominantly Ordovician, the other dominantly Silurian. An early view (McKerrow et al. 1977; Leggett et al. 1982) that the Ordovician and Silurian rocks represent an accretionary prism has been challenged by suggestions that it is a thrust-stacked sedimentary basin once marginal to an arc (Stone et aL 1987). The various fault slices are dominated by turbiditic greywackes, black and grey pelagic and hemipelagic mudstones together with cherts and some pillow lava: basinal (or trench) and open oceanic environments are probably represented. In the northern and central belts, the base of the greywacke in successive slices becomes progressively younger in a southeasterly direction. Greywacke clasts include mostly igneous and metamorphic detritus, with igneous clasts having both ophiolitic and arc-like provenance. Southerly derived fresh calc-alkalic detritus in the northern belt has yielded Cambrian ages so that at least some of this material was not eroded from any contemporaneous arc but involves an old outboard arc, now lost (Kelley & Bluck 1989; Styles et al. 1989, see also Bluck et al. 1989). The relationship between the Southern Uplands block and the Midland
2
TERRANES
Valley terrane remains uncertain. If the northern belt of the Southern Uplands represents an Ordovician accretionary prism, and the south Midland Valley a proximal fore-arc, then considerable movement along the Southern Uplands Fault is required to remove most of any original fore-arc basin. The presence of Silurian fans with abundant quartzite and igneous clasts in the south Midland Valley close to the Southern Uplands Fault suggests a source block in the position of the Southern Uplands that has now been removed. The structural style of the mid Ordovician to Early Silurian rocks at Girvan, north of the Southern Uplands Fault, is indicative of later Silurian pressure from the southeast producing asymmetrical folds and northwest-directed thrusting. This, together with geophysical evidence which indicates that a metamorphic basement lies at no great depth beneath the Southern Uplands (Hall et al. 1983), suggests that the latter is allochthonous and that strike-slip movements along the Southern Uplands and associated faults have no more than complicated the relationship. The Iapetus, or Solway, line is concealed by Upper Palaeozoic sediments in northern England but in parts of eastern and central Ireland is believed to be represented by the Navan-Silvermines Fault. Although seismic profiles show northwesterly dipping reflectors defining the deep crustal expression of the suture (Leggett et al. 1983), its present surface expression is evidently a post-closure feature (approximately Wenlock onwards) when a r6gime dominated by transpressional convergence became established. Faunal connections were becoming established by Ashgiil times and it is evident, particularly in central Ireland, that the Silurian successions on either side of the line belong to the same residual basin and separation must then have been small. The striking faunal and lithostratigraphical characteristics which so clearly separate the terranes to either side of the suture are confined to Cambro-Ordovician sequences. Laurentian to the north, Gondwanan to the south. Devonian lavas and plutons occur to either side of the Iapetus line. A small suspect terrane, immediately to the south of the Navan Fault in east Ireland, which has puzzled a number of workers for some time is that termed Grangegeeth terrane (Harper & Parkes 1989). This tract contains a partly volcanic Ordovician sequence which yields 'Atlantic' (i.e. southern) graptolites from the Lower Ordovician but the lower Caradoc yields shelly faunas which are dominantly Laurentian in aspect. It may be that the Grangegeeth block represents an intermediate terrane, once situated within the southern Iapetus Ocean at a fairly high southern latitude where it was influenced largely by Gondwanan climatic and faunal patterns, but which subsequently, with the progressive closure of Iapetus, came with the influence of Laurentia. It is likely that this terrane has been substantially relocated by strike-slip movements. South of the Southern Uplands, Lower Palaeozoic rocks exposed in the Lake District-Howgill Fells, Cross Fell, Teesdale and Craven inliers, Isle of Man and the Leinster Massif in east Ireland are here all correlated as belonging to one Leinster-Lakesman suspect terrane. It has not been proven whether this terrane was contiguous throughout the Lower Palaeozoic with other areas to the south (e.g. Welsh Basin) but faunal ties are particularly strong by late Ordovician times. Although Cambrian strata are suspected from eastern Ireland, the oldest proven strata in the Isle of Man and the Lake district are of late Tremadoc age. The Lower and Middle Ordovician succession is dominated by marine siliciclastic sediments and subduction related talc-alkaline volcanic rocks, overstepped in the Late Ordovician by calcareous mudstones and limestones. The marine basin which developed during the Silurian shows lithostratigraphical similarities with both Welsh Basin and Southern Uplands rocks of similar age. The limited amount of subsurface information from eastern England suggests that the concealed Caledonides there, partly of offshore and volcanic aspect, may well represent a southeastwards continuation of the Leinster-Lakesman terrane. The Lower Palaeozoic rocks exposed in Wales and southern Britain generally were deposited on a late Precambrian basement that includes at least one major suspect terrane boundary--the Menai Strait Fault System. To the northwest of this fault system Lower Palaeozoic rocks rest unconformably upon late Precambrian 'Monian' rocks which include calc-alkaline igneous rocks, sediments with andesitic detritus, blueschists and m~langes indicating proximity to a late Precambrian to earliest Cambrian subduction zone of unknown polarity (Gibbons 1983, 1990a). Undated gneisses in central Anglesey are associated with a 600 Ma granite, both of which are in tectonic contact with low grade Monian Supergroup metasediments. The latter are probably of latest Precambrian to earliest Cambrian age and have been correlated with the Cullenstown Formation of southeast Ireland (Tietzsche-Tyler & Phillips 1989). The blueschists, yielding an Ar-Ar uplift age of c. 550 Ma (Dallmeyer & Gibbons 1987), occur only as a transcurrently transported sliver within the Menai Strait Fault System. To the southeast of this system Precambrian rocks in Wales and England are characterized by calc-alkaline igneous and marine-fluviatile sediments derived from them: they are typically 'Avalonian' in character and are interpreted as having formed part of the same Late Proterozoic arc system as the rocks of the Avalon Peninsula of east Newfoundland and similar rocks elsewhere in maritime eastern North America. Similarly, calc-alkaline igneous rocks in the Channel Islands (referred to as Cadomian) are presumably part of the same active plate margin although they intrude > 2000 Ma lcartian gneisses and low grade Brioverian metasediments, neither of which is seen on the British mainland (Gibbons 1990b). They now form part of the North Armorican terrane of the Variscides. The Lower Palaeozoic cover sequence immediately southeast of the Menai Strait Fault System comprises a thick (> 10 kin) sequence of marine clastic
sediments (Welsh Basin) that has been divided into three supergroups each entirely or largely underlain by a basinwide unconformity (Woodcock 1990). A Lower Cambrian through Tremadoc Dyfed Supergroup begins, in the north, with acid voicanicity (Arfon Group) and ends with the eruption of ensialic arc volcanics. It is likely that the post-Dyfed angulai" unconformity was volcanotectonic in origin. An overlying Arenig to Caradoc Gwynydd Supergroup shows a trangressive base, progressing into offshore and basinal mudstones with oceanic connections, and is further characterized by the development of many ensialic, supra-subduction volcanic centres, mostly Llanvirn in the south, Caradoc in the north. The Ashgill to Lower Devonian Powys Supergroup essentially records a cessation of volcanicity (as in the Leinster-Lakesman terrane), pre-Upper Llandovery reactivation of transcurrent basin margin faults and renewed margin sedimentation, finally ending in a regressive basin-fill sequence and the deposition of red beds. The Welsh Basin is bounded to the southeast by the Welsh Borderlands Fault System (Woodcock & Gibbons 1988), involving the Pontesford Lineament and the Church Strettoh Fault belt, beyond which the Lower Palaeozoic succession on the Midlands Platform (microcraton) is generally much thinner, less complete, and was deposited in shallower water. The Dyfed Supergroup is represented largely by up to 1.5 km of shallow marine Cambrian clastics but with a substantial thickness of Tremadoc mudstones: it rests unconformably on the late Precambrian basement. The Gwynydd Supergroup is represented only by marine sandstones and mudstones of Caradoc age ( < 800 m), whereas the Powys Supergroup oversteps the Welsh Borderland Fault System as a series of Silurian shallow marine siliciclastics and limestones overlain by late Silurian and Lower Devonian red beds. The paratectonic Variscides exposed in southwest England involve partly basinal Devonian to Carboniferous siliciclastics with some limestones and basic volcanics. It is thought that they conceal, at depth, a southern extension of the Avalon terrane, composed substantially of Precambrian metamorphic and volcanic rocks and which supplied detritus to the Welsh Basin during much of the Early Palaeozoic (Cope & Bassett 1987; Cope 1988). In the south, close to the Lizard terrane, Middle--Upper Devonian olistostromes in the Carrick nappe contain phacoids of Llandeilo Gorran Quartzite, of Armorican aspect, together with Lower and Middle Devonian rocks (Sadler 1974; Holder & Leveridge 1986). The Lizard terrane consists of a dismembered ophiolite complex, mostly of peridotites, gabbros and sheeted dykes in tectonic contact with amphibolites and metasediments of probable, but unproven Mid-Late Devonian age. Some of the Lizard amphibolites define ductile shear zones interpreted as having formed in an oceanic setting during extensional dismemberment of a slow-spreading ridge (Gibbons & Thompson 1991). This Lizard Complex was thrust over the paratectonic Variscides during the advance of the 'Normannian High' in late Devonian and Carboniferous times. BJB, WG, JKI
References BENTLEY,M. R., MALTMAN,A. J. & FITCnES, W. R. 1988. Colonsay and lslay: a suspect terrane within the Scottish Caledonides. Geology, 16, 26-28. BLUCK, B. J. 1985. The Scottish paratectonic Caledonides. Scottish Journal of Geology, 21,437-464. - - - , ELDERS,C. F., FLOYD,J. D., KELLEY,S. & STONE,P. 1989. Discussion on detrital mineral ages from the Southern Uplands using 4~ laser probe. Journal of the Geological Society, London, 146, 401-403. , 1NGHAM,J. K., CURRY, G. B. & WILLIAMS,A. 1984. VI. Stratigraphy and tectonic setting of the Highland Border Complex. In: CURRY,G. B., BLUCK, B. J., BURTON,C. J., INGHAM,J. K., SIVETER,D. J. & WILLIAMS,A. (eds) Age Evolution and Tectonic History of the Highland Border Complex, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 75, 113-133. - - & LEAKE,B. E. 1986. Late Ordovician to Early Silurian amalgamation of the Dalradian and adjacent Ordovician rocks in the British Isles. Geology, 14, 917919. CANNAT, M. 1989. Late Caledonian north-eastward ophiolite thrusting in the Sheltand Islands, U.K. Tectonophysics, 169, 257-270. COPE,J. C. W. 1988. Pre-Devonian history of south-west England. Proceedingsof the Ussher Society, 7, 468-473. - - & BASSETT,M. G. 1987. Sediment sources and Palaeozoic history of the Bristol Channel area. Proceedingsof the Geologists' Association, 98, 315--330. DALLMEVER,R. D. & GIaSONS, W. 1987. The age of blueschist metamorphism in Anglesey, North Wales: evidence from 4~ mineral dates of the Penmynydd schists. Journal of the Geological Society, London, 144, 843-852. DEMPSTER,T. J. & BLUCK,B. J. 1991. The age and tectonic significance of the Bute amphibolite, Highland Border Complex, Scotland. Geological Magazine, 128, 77-80. DEWEY, J. F. & PANKHURST,R. J. 1970. Evolution of the Scottish Caledonides in relation to their isotopic age pattern. Transactions of the Royal Soc&ty of Edinburgh, 68, 361-389. ELL~OTT,D. & JOHNSON,M. R. W. 1980. Structural evolution in the northern part of the Moine thrust belt, N. W. Scotland. Transactions Of the Royal Society of Edinburgh. Earth Sciences, 71, 6%-96. FITCHES,W. R. & MALTMAN,A. J. 1984. Tectonic development and stratigraphy at the western margin of the Caledonides: Islay and Colonsay, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 75, 365-382. GmBONS, W. 1983. Stratigraphy, subduction and strike-slip faulting in the Mona Complex of North Wales--a review. Proceedingsof the Geologists' Association, 94. 147-163. -1990a. Pre-Arenig terranes of NW Wales. In: STRACHAN,R. A. & TAYLOR, G. K. (eds) Avalonian and Cadomian geology of the North Atlantic margin. Blackie, Glasgow, 28-48. -1990b. Transcurrent ductile shear zones and the dispersal of the Avalon Superterrane. In: D'LEMoS,R. S., STRACHAN,R. A. & TOPLEY,C. G. (eds) The Cadomian Orogeny. Geological Society, London, Special Publication. 51. 407423.
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
4
TERRANES
& THOMPSON, L. 1991. The Porthoustock Shear Zone: oceanic mylonitisation in the Lizard ophiolite. Proceedings of the Ussher Society, abstract, in press. HALL, J., POWELL, D. W., WARNER, M. R., EL-ISA, Z. H., ADESANYA,O. & BLUCK, B. J. 1983. Seismological evidence of shallow crystalline basement in the Southern Uplands of Scotland. Nature, 305, 418-420. HARPER, D. A. T. & PARKES, M. A. 1989. Short Paper: Palaeontological constraints on the definition and development of Irish Caledonide terranes. Journal of the Geological Society, London, 146, 413 ~,15. HOLDER, M. T. & LEVERIDGE,B. E. 1986. A model for the tectonic evolution of south Cornwall. Journal of the Geological Society, London, 143, 125-134. HUTTON, D. H. W., AFTALION,M. & HALLIDAY,A. N. 1985. An Ordovician ophiolite in County Tyrone, Ireland. Nature, 315, 210-212. INGHAM, J. K., CURRY, G. B. & WILLIAMS, A. 1986. Early Ordovician Dounans Limestone fauna, Highland Border Complex, Scotland. Transactions of the Royal Society of Edinburgh." Earth Sciences, 76(4) (for 1985), 481-513. KELLEY, S. & BLUCK, B. J. 1989. Short Paper: Detrital mineral ages from the Southern Uplands using 4~ laser probe. Journal of the Geological Society, London, 146, 401-403. LEAKE, B. E. 1963. The Location of the Southern Uplands Fault in Central Ireland. Geological Magazine, 100, 420-423. LEGGETT, J. K., MCKERROW, W. S. & CASEY, D. M. 1982. The anatomy of a Lower Palaeozoic accretionary fore-arc: the Southern Uplands of Scotland. In: LEGGETT, J. K. (ed.) Trench-Forearc Geology. Geological Society, London, Special Publication, 10, 495-520. --, - & S O P E R , N. J. 1983. A model for the crustal evolution of Southern Scotland. Tectonics, 2, 187-210. MCKERROW, W. S., LEGGETT,J. K. & EALES,M. 1977. Imbricate thrust model of the Southern Uplands of Scotland. Nature, 267, 237-239. RODDAM, D. S. & MILLER, J. A. 1989. An early phase in the closure of Iapetus: the age of the pyroxene-garnet-hornblende schist associated with the Shetland ophiolite. Terra Abstracts, l(l), 9.
ROGERS, G.. DEMPSTER, T. J.. BLUCK, B. J. & TANNER, P. W. G. 1989. A high precision U-Pb age for the Ben Vuirich granite: implications for the evolution of the Scottish Dalradian Supergroup. Journal of the Geological Socieo', London, 146, 789 978. SADLER, P. M. 1974. Trilobites from the Gorran Quartzites, Ordovician of south Cornwall. Palaeontology, 17, 71-93. STONE, P., FLOYD, J. D., BARNES,R. P. & LmTERN, B. C. 1987. A sequential back-arc and foreland basin thrust duplex model for the Southern Uplands of Scotland. Journal o/" the Geological Society, London, 144, 753-764. STYLES, M. T., STONE, P. & FLOYD, J. D. 1989. Short Paper: Arc detritus in the Southern Uplands: mineralogical characterization of a 'missing" terrane. Journal of the Geological Society, London, 146, 397-400. TANNER, P. W. G., DEMPSTER, T. J. & DICKIN, A. P. 1989. Short Paper: Time of docking of the Connemara terrane with the Delaney Dome Formation, western Ireland. Journal of the Geological Society, London, 146, 389-392. TIE'rZSCH-TYLER, D. & PHILLIr'S, E. 1989. Correlation of the Monian Supergroup in NW Anglesey with the Cahore Group in SE Ireland. (Extended Abstract).
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Journal of the Geological Society, London, 146, 417-418. WArSON, J. 1983. Lewisian. In CRAIG, G. Y. (ed.) Geology of Scotland. Scottish Academic Press, Edinburgh. WILLIAMS,D. M., ARMSTRONG,H. A. & HARPER, D. A. T. 1988. The age of the South Connemara Group, Ireland and its relationship to the Southern Uplands Zone of Scotland and Ireland. Scottish Journal of Geology, 24, 279-287. WOODCOCK, N. H. 1990. Sequence stratigraphy of the Palaeozoic Welsh Basin. Journal of the Geological Society, London, 147, 537-548. & GmaoNs, W. 1988. ls the Welsh" Borderlands Fault System a terrane boundary? Journal of the Geological Society, London, 145, 915-923.
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Precambrian R. A N D E R T O N , W. GIBBONS & P. G. N I C H O L S O N
Northwestern British Isles: Upper Proterozoic For much of Precambrian time, it is not possible to reconstruct any sort of meaningful palaeogeographies. For this Atlas, therefore, the earliest reconstructions are for Proterozoic intervals and are, of necessity, limited geographically. The term 'Torridonian' has long since been used to refer to the entire Upper Proterozoic succession of predominantly fluvial clastic sediments situated along the northwest coast of Scotland. These rocks rest unconformably on Archaean to Lower Proterozoic Lewisian Gneiss, are overlain unconformably by Cambro-Ordovician marine sediments, and constitute the best sedimentary exposures in the British Isles. Stratigraphically, 'Torridonian' deposits are subdivided into the Stoer, Sleat and Torridon groups. The Stoer and Torridon groups are separated by an angular unconformity (Lawson 1965; Stewart 1969) and have been dated at 968 and 777 Ma respectively (Moorbath 1969; Stewart 1982), although these ages may be up to 100 Ma too young (see discussion in Stewart 1988a, p. 98). Sedimentation dates of c. 1050 (Stoer Group) and c. 850 (Torridon Group) are, therefore, likely to be more realistic.
PCIa: Upper Proterozoic Stoer Group The lowest of the three groups consists of a diverse suite of red bed deposits up to 2 km thick representing fluvial, aeolian and ephemeral lacustrine environments, suggesting a semi-arid climate (Stewart 1988a). Deposition occurred at an approximate palaeolatitude of 15~ (Torsvik & Sturt 1987). Stoer Group sediments have been divided by Stewart (1988a) into seven constituent facies. The illustrated palaeogeographical time interval encompasses the end of Bay of Stoer facies deposition, the emplacement of the Stac Fada Member and the onset of Poll a' Mhuilt Member sedimentation (the latter defined by Upfold 1984a). The Bay .of Stoer facies consists of roughly 200m of cross-bedded sandstone with sporadic pebbles, deposited by braided rivers flowing from west to east (present day) across the basin. This period of fluvial sedimentation ended abruptly with the emplacement of the Stac Fada Member, a 12 m thick horizon of volcanic breccia with a muddy sandstone matrix that has been interpreted as an ashflow (Lawson 1972), a volcanic mudflow (Stewart 1978), and more recently as a phreatomagmatic peperite slurry (Sanders & Johnston 1989). Movement on the north south trending Coigach Fault may have triggered Stac Fada magmatism. Relative easterly downthrow on this fault disrupted the Bay of Stoer facies' fluvial drainage pattern, and also created a small intra-rift basin in which Lake Poll a' Mhuilt began to form (Upfold !984a). Intercalations of wave-rippled siltstones and mudstones with algal mat limestones were laid down directly on top of the Stac Fada Member during these initial lacustrine stages (Upfold 1984a,b). The Stac Fada Member forms a key correlative horizon, as it can be traced along almost the entire north-south length of the Stoer Group outcrop belt. A restored cross-section constructed by Lawson (1970) along strike indicates that this member terminates against the Loch Maree Fault to the south, thereby suggesting a higher relative structural position for this "southern' crustal block during the early part of Stoer deposition. The Loch Maree Fault itself is a major strike-slip displacement that was reactivated along one of several northwest-southeast trending Early Proterozoic shear zones within the Lewisian basement. Field relationships show that most of the present displacement along this fault is of Precambrian origin. According to Stewart (1982, 1988a), Stoer Group sedimentation occurred within a rift graben bounded to the west by the Minch Fault and to the east by a normal fault lying near the present Moine Thrust zone. Detritus of predominantly Scourian provenance (c. 2600 Ma) was supplied to the basin from the flanking horst blocks (Stewart 1982, 1988a). Two 180 ~ palaeocurrent reversals within the succession are thought to represent fault-controlled tilting of the basin floor, coupled with concomitant source-block rejuvenation on either side of the rift (Stewart 1982, 1988a). Although the Stoer basin was probably bounded to the west by normal movement on the Proterozoic Outer Isles Fault (Stein 1988; Lailey et al. 1989), evidence for a similar normal fault along the eastern basin margin is tenuous. Moreover, Stewart's rift-graben model fails to address the possible existence of a basin-axial drainage system, which is a common feature of 9 1992The GeologicalSociety
many extensional depositional settings. Consequently, even though a general rift environment seems valid for the Stoer Group, it is likely that the Loch Maree Fault, together with other northwest-southeast trending (shear zone induced) displacements, had a far greater control on Stoer Group basin genesis and sedimentation than hitherto recognized. PGN
Upper Proterozoic Sleat Group Found only within the Kishorn Nappe of Skye is the 3.5 km thick Sleat Group, consisting of coarse grey fluvial sandstones and subordinate grey lacustrine/shallow marine shales which together have suffered lower greenschist metamorphism (Stewart 1988b). These sediments pass upwards. apparently conformably (Stewart 1988b), into overlying Torridon Group deposits. Sleat palaeocurrents are predominantly from the west (Sutton & Watson 1964). Despite the apparently conformable relationship between the Sleat and Torridon groups, Stewart (1982, 1988b) points out that discrepancies exist between their pebble suite and detritus compositions. He has suggested that these differences indicate Sleat deposition in a separate basin (sub-graben). Similarly, sediments within the Tarskavaig Nappe (the 'Tarskavaig Moines') are also interpreted by Stewart (1988b) to have formed in an independent trough. The palaeogeographical positions of the Sleat and Tarskavaig basins, together with their lateral relationships to Torridon Group sediments on the foreland (and to the Moine?), remain enigmatic. PGN
P~Ib: Upper Proterozoic Torridon Group Dominating the 'Torridonian' outcrop belt are the coarse red fluvial arkoses of the Torridon Group. This group reaches a maximum stratigrapbical thickness of 5-6km, and is subdivided into Diabaig (0.6km maximum thickness), Applecross (3.0-3.5 km thick), Aultbea (1.5-2.0km thick) and Cailleach Head (0.8 km thick) formations (Stewart 1988b). The vast majority of outcrops belong to the Applecross Formation, including most of the spectacular coastal mountains of northwest Scotland. Situated stratigraphically below but with a diachronous relationship to the Applecross Formation are the grey shales and siltstones (and local breccias and sandstones) of the Diabaig Formation. During the initial stages of deposition, when Applecross rivers first entered the Torridon Group basin from the west, isolated Diabaig lakes formed in topographic lows on the Lewisian Gneiss basement. Diabaig lakes were soon replaced by fluvial conditions as Applecross rivers began to prograde across the basin, eventually filling the Lewisian valleys and then burying the remaining hills. Applecross sediments range from matrix-supported pebble conglomerates in the north, to coarse sandstones with a variable pebble content in the south. Fluvial sedimentation persisted for a substantial period of time, producing what is essentially a 5-6 km thick mega-scale fining-upwards succession from coarse pebbly Applecross arkoses through to more homogeneous medium-grained Aultbea sandstones. The overlying grey shale-to-sandstone cyclothems of the Cailleach Head Formation are thought to represent freshwater deltaic deposits (Stewart 1988b). Recent studies of the Applecross Formation by Nicholson (1989 and MSS) have yielded new insights into the deposition of this alluvium and ultimately the overall genesis of the Torridon Group. Applecross sandstone bodies exhibit an abundance of internal cross-stratification and an almost total absence of evidence for sheetflooding, implying a dominance of simple channel sedimentation. In addition, large-scale bar structures (typically 3 5m thick but reaching up to 9m) are frequently recognizable, indicating the existence of major rivers within the depositionai system (Nicholson 1989 and MSS). Collectively, these features suggest deposition on a major alluvial braidplain, as opposed to the alluvial fans originally proposed by Williams (1966). Detailed basin-wide palaeoflow analyses by Nicholson (MSS) demonstrate that mean local palaeoflow vectors, although 'skewed' across the basin as shown for lower-middle Applecross sediments, do not exhibit radial patterns as suggested by Williams (1966, 1969a). A warm, humid climate during deposition of the Applecross Formation was favoured by Williams (1968, 1969a). Stewart (1988b) stated that the
6
PRECAMBRIAN
Torridon Group climate was much wetter than that of the Stoer Group based on comparative overall petrography and geochemistry, while Torsvik & Sturt (1987) indicated that Applecross palaeolatitude was roughly 35S. The ubiquitous cross-stratification within the Applecross alluvium, together with the evidence for major rivers, also indicates a wet, possibly sub-tropical depositional environment (Nicholson 1989 and MSS), as does the total absence of caliche, evaporites or aeolian deposits (in obvious contrast to the Stoer Group). The concept of depositional system scale is crucial to the palaeogeographical interpretation of Torridon Group genesis. According to Stewart (1982, 1988a,b), the Torridon Group (like the Stoer Group) was deposited in a fault-bounded rift basin, with. the flanking horst areas acting as sediment source in the west (Minch Fault scarp-sourced alluvial fans) and basin margin in the east (basin-bounding normal fault lying near the present Mome Thrust Zone). However, certain Applecross characteristics cannot be reconciled to such a model. A 200 km wide outcrop belt, with recurring evidence for major rivers and a uniformly east-southeast regional palaeoflow, indicates that the depositional basin was larger in scale than a relatively self-contained rift graben model, and that the source hinterland was probably substantially more distant than the neighbouring Outer Hebrides (Nicholson MSS). Unlike the Stoer Group, the Applecross and Aultbea formations' uniformity of both petrography and facies (a 5 - 6 k m thickness of cross-stratified sandstone with a variable pebble content) further supports a depositional system scale greater than that generated from an immediately flanking western horst block and deposited by an inter-rift drainage network. Regional palaeoflow data (Nicholson MSS), as well as failing to confirm deposition by scarp-generated alluvial fans, also show no evidence for longitudinal rivers or axial drainage patterns along the eastern edge of the outcrop belt, as might be expected had there been a basin-bounding normal fault in the vicinity. These scale considerations imply that the Outer Isles Fault, despite bounding the present-day western limit of the 'Torridonian' (Stein 1988; Lailey et al. 1989), was unlikely to be a controlling (basinbounding) feature of Torridon Group deposition (Nicholson MSS). Rogers et al. (1989) have identified Grenvillian (c. 1100 Ma) and Archaean (c. 2700 Ma) detrital zircons within the Applecross lithosome, together with the Laxfordian (c. 1600 Ma) detritus suggested by Stewart (1982, 1988b) to be the sole Applecross sediment source. These detrital zircon results also support the likelihood of a larger drainage area and accordingly more distal source (further onto the Laurentian Shield). In addition, much of the Applecross pebble suite has been matched with crustal blocks in Greenland and Labrador (Williams 1969b; Allen et al. 1974). Although Cliff & Rex (1989) have found Grenville cooling ages in biotites from the Lewisian Complex on Harris and Lewis, it is uncertain whether this area was actually involved in a major Grenvillian detritus-forming event. The illustrated palaeogeographical reconstruction of Torridon Group sedimentation with an apparently unconstrained easterly palaeoflow, coupled with the previously mentioned age uncertainties, once again raises the question of a prospective (if only partial) 'Torridonian'-Moine correlation. PGN
Moine Supergroup The Moine Supergroup defines a metamorphic complex in the Northern Highlands of Scotland, cropping out between the Great Glen Fault and the Moine Thrust Zone (Harris & Johnson 1991). Moine rocks lie in the hanging wall of the Moine Thrust, having been thrust northwestwards over the Lewisian basement and overlying Torridonian and Cambro-Ordovician cover. Moine lithologies are mostly psammites and pelites of amphibolite grade (locally greenschist), with basic amphibolites and minor calc-silicate horizons. They have a complicated tectonometamorphic history derived from the overprint of Lower Palaeozoic (Caledonian) deformation on rocks already metamorphosed by Precambrian events. Interleaved with the Moine Supergroup are slices of reworked Lewisian basement. The Lewisian/Moine contact is normally tectonic (Rathbone & Harris 1979), although deformed conglomerates interpreted as marking an original unconformity occur locally e.g. Loch Carron in Inverness-shire and near the Kyle of Tongue in Sutherland (Peach et al. 1910; Ramsey 1958; Allison et al. 1989). Despite the dismemberment of the complex by ductile shear zones and the geometric complexities induced by polyphase folding, detailed work has produced a well defined lithostratigraphic subdivision of the Supergroup into three groups (Johnstone et al. 1969; Johnstone 1975; Roberts et al. 1987; Harris & Johnson 1991). These three units are the Morar Group (subdivided into four formations of psammite and pelite), the Glenfinnan Group (laterally varied but including three pelite/psammite formations), and the Loch Eil Group (mostly psammites). A recognizable stratigraphical succession using cross-bedded psammites is well documented in the Morar Group which, in the Mallaig-Arisaig district is estimated as between 6 and 7 km thick, although the lowest formation (Basal Pelite) is highly tectonized with urlderlying Lewisian basement. Lithological variation within the higher grade Glenfinnan Group is complicated by tectonic overprinting and probably by original lateral facies changes. Over most areas, the Morar/Glenfinnan contact is a major ductile thrust (the Sgurr Beag shear zone), although in the Ross of Mull the Morar Group shows an apparently normal stratigraphical contact with the Glenfinnan Group (Holdsworth 1987). Uncertainty exists over whether part of the Glenfinnan Group may have originally overstepped on to Lewisian basement in" the east. The Loch Eil
Group stratigraphically overlies the older Glenfinnan Group (Roberts & Harris 1983; Roberts et al. 1984). The age of Moine Supergroup deposition is poorly constrained. A maximum age is provided by the Lewisian basement, eclogites within which have been dated as c. 1100 Ma (Sm/Nd) (Sanders et al. 1984). Determination of a minimum age relates to the likelihood that the Moine Supergroup is older than Dalradian rocks which were deformed in late Precambrian times (590 Ma U-Pb; Rogers et al. 1990). Moine rocks show broad similarities to the 'Central Highland Granulites' that underlie the Dalradian Supergroup southeast of the Great Glen Fault. Ongoing research on U-Pb dating of the Moine metasediments is throwing doubt upon the validity of supposedly Grenvillian dates once obtained from the Moine metasediments (Brook et al. 1976; Rogers, pers. comm.). The 'Central Highland Granutixes' have been the subject of controversy concerning whether they lie unconformably beneath, or pass up without a stratigraphic break into, the overlying Dalradian (Haselock & Gibbons 1990; Harris & Johnson 1991). Moinian and Central Highland rocks have yielded a spectrum of Rb-Sr pegmatite ages of 780-730Ma and 650-630Ma (Piasecki 1980; Piasecki & Van Breeman 1983), but the significance of these ages still remains uncertain. A poorly constrained but probably early Upper Proterozoic age for Moine sedimentation keeps alive Geikie's original suggestion in 1895 that there may be a correlation between Moine and Torridonian. The possibly coeval Moine would represent the offshore equivalent of the Torridonian red beds. This possibility has been much debated (e.g. Johnstone 1975), especially as the metasediments in the Tarskavaig nappe of Skye appear transitional in character between Torridonian in the footwall of the sole thrust and Moine in the hanging wall of the overlying Moine Thrust (Stewart 1975). WG
PC2: Northwestern British Isles: Upper Proterozoic Dalradian Supergroup The Dalradian Supergroup is a thick and extensive succession, largely of metasediments but with minor metavolcanics and intrusions, that stretches across the northern British Isles from Shetland to Connemara. It was deposited during a period of over 150 Ma starting in late Proterozoic times, perhaps before 750 Ma ago, and ending before 600 Ma ago. As the rocks have been variably deformed and metamorphosed to the greenschist or amphibolite facies, original sedimentary detail, on which palaeogeographical reconstructions can be based, is sparse and tends to be common only in southwest Scotland and northeastern Ireland. All the reconstructions presented here are therefore highly speculative and based on extrapolations from observations made at a limited number of localities. The position and nature of the base of the Dalradian pile is controversial (cf. Harris et al. 1978; Piasecki et al. 1981). It is likely that the onset of Dalradian sedimentation marks the beginning of a new phase of lithospheric stretching, felt throughout the Caledonian region, that was subsequently to lead to the fragmentation of the Proterozoic supercontinent (Piper 1983) and the formation of some Palaeozoic oceans. This phase of stretching postdated the Grenville orogenic events which were responsible for the initial deformation of the Moine terrane. Thus, the onset of Dalradian sedimentation was probably marked by the transgression of a late Precambrian sea across a Proterozoic (Moine + Torridon + Lewisian + other units no longer preserved) basement. The originally unconformable base of the Dalradian may now be difficult to recognize because of subsequent sliding (Soper & Anderton 1984). The restrictions of the Dalradian outcrop pattern, resulting from the Athollian and Grampian phases of Caledonian folding in the late Precambrian-earliest Ordovician, mean that facies changes in a northeast-southwest direction, parallel to the direction of the presumed coastline, are better known than onshore-offshore changes in a NW-SE direction. The tidalshelf quartzite horizons (Eilde Binnean, Glen Coe, Jura) fine to the northeast (Hickman 1975; Anderton 1976) in what is presumed to be the coastlineparallel ebb tidal current transport direction. Petrographic data suggest that the bulk of the Dalradian detritus was derived from source lands to the northwest (Anderton 1980). The most likely palaeogeographical picture for the general area of Dalradian deposition, then, is in the northwestern sector of a major marine gulf that advanced southwestwards along what subsequently became the trend of the Caledonian belt. The gulf would have been funnel-shaped, opening onto an ocean basin at some distance to the northeast. The gulf is thought to have been ensialic in late Proterozoic times with sediment input laterally from the northwest and southeast and axially from the southwest. Lithospheric stretching led to local volcanism and the development of quasi-oceanic crust before the final failure of the crust and the onset of sea-floor spreading, at the end of the Proterozoic, rent the gulf in two. The northwestern sector of the gulf became part of the continental margin of the Laurentia and the southeastern sector became part of another palaeocontinent, perhaps, but not necessarily, Baltica. The inset on P~ 2a shows the areas of late Proterozoic sedimentation on a continental refit loosely following Patchett et al. (1978). The figure itself shows a generalized Grampian Division to Lochaber Subgroup palaeogeography for the whole Dalradian area. It has been drawn on the assumption that the formations seen in southwest Scotland, would, if traced far enough, be seen to be lateral facies equivalents, as implied by Hickman (1975) and Harris et al. (1978). PC 2b and 2c have been drawn on the same assumption with the facies belts named according to the formations they are supposed to
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
8
PRECAMBRIAN
represent. It is emphasized that these palaeogeographies are in no way "snapshots' of a particular moment in time, but are conceptual models illustrating possible relationships between the environments of deposition of various formations. P-E 2a shows the character of the Dalradian environment for some time after the initial marine transgression down the axis of the gulf. Abundant clastic sediment was supplied laterally and axially. Tidally dominated deltas were being scoured out to form estuarine channels with fines deposited on adjacent tidal flats. Sediment was being transported by tidal currents down the gulf approximately parallel to the coast with a high energy sand facies passing downcurrent into lower energy silts and muds. Migration of these facies belts in response to changes in clastic input, tidal energy and shoreline position could have produced the stratigraphical sequence we see from the Grampian Division and Lochaber Subgroup, up to the lower part of the Leven Schists. The Upper Leven Schists seem to record an overall decline in environmental energy due to basin deepening before the re-advance of clastic sediments produced the Batlachulish Subgroup pattern of P s 2b. Maps Ps 2b-i are enlargements of the boxed area on P s 2a. P~ 2b has many similarities to the previous map but includes the offshore deep water carbonate facies of the Ballachulish Limestone. Blair Atholl Subgroup times (PC 2c) are marked by a decline of clastic input and the establishment of a poorly defined carbonate shelf with area of ?deeper water clastic mud accumulation. The Argyll Group maps (P~ 2d-i) are a little more detailed and less generalized. Following the advance of the ice, which led to the deposition of the Port Askaig Tillite (PC 2d), the area returned to carbonate shelf conditions and the Bonahaven Dolomite accumulated (PC 2e). A renewed phase of clastic input and tidal energy formed the Jura Quartzite (PC 2f) until the subsidence of fault-bounded blocks broke the shelf into a series of turbidite basins (PC 2g). These basins gradually filled up, allowing the establishment of shallow to marginal marine conditions (PC 2h) before renewed subsidence produced the Crinan Grit turbidite basins (PC 2i). The rate of crustal stretching and basin subsidence, although proceeding in fits and starts throughout the Grampian Division and Appin Groups, had accelerated by late Argyll Group times to the point at which quasi-oceanic crust was developing by a process of sill injection. This magmatism culminated at the beginning of Southern Highland Group times in the eruption of the Tayvallich Volcanics (P~ 2j) possibly in one of a series of small pullapart basins. However, these did not develop further into true ocean crust and final continental rupture may have taken place along a line to the southeast nearer the centre of the original gulf. Throughout most of the Cambrian the Dalradian tract was a zone of crustal thickness as deformation and metamorphism progressed. RA
P ~ 3 a, b Southern British Isles: late Precambrian The Precambrian basement to southern Britain is mostly young (
Wales
The oldest rocks exposed in Anglesey and on the L12~npeninsula were first described as Monian by Blake (1888), and later defined as the Mona Complex by Greenly (1919). The area contains highly disparate geological units that show only tectonic boundaries with one another and has been interpreted as a series of suspect terranes (Gibbons 1983a, 1989). At least four such terranes may be recognized: the Monian Supergroup (Shackleton 1975), the Central Anglesey gneisses and Coedana Granite (Coedana Complex), the southeast Anglesey blueschists and the Sarn Complex. The lower part of the low grade, polydeformed Monian Supergroup (South Stack Group) is psammitic and contrasts with the overlying, more pelitic New Harbour Group which contains minor serpentinites, metagabbros and metabasalts with a geochemical character suggestive of a subduction-related (arc) origin (Thorpe et al. 1984). The upper part of the Monian Supergroup is dominated by a regional-scale melange (Gwna M6lange) containing fragments of igneous rocks and various deep- and shallow-water sediments. The Cuilenstown Formation metasediments in southeast Ireland have been correlated with the South Stack Group (e.g. Tietzsch-Tyler & Phillips 1989). Palaeontological data for the Monian Supergroup are poor: stromatolites in Gwna M61ange limestone clasts in northern Anglesey have been ascribed a late Precambrian or Early Cambrian age (Wood & Nicholls 1973). Possible Skolithos burrows in the South Stack Group, and filamentous microfossils of uncertain age in jasper from the Gwna M61ange provide the only other palaeontological data (Muir et al. 1979; Peat 1984). An original report of Lower Cambrian acritarchs (Muir et al. 1979) has not been confirmed by later study of the same material by Peat (1984). The undated gneisses of central and northeast Anglesey include silli-
manite-bearing migmatitic pelites and amphibolites. They are closely associated with the c. 600 Ma Coedana Granite and its associated hornfels, although the relationships between all of these units remain uncertain. The northwest boundary of this central Anglesey area is a post-Arenig brittle fault zone, whereas the southeast boundary is a wide, ductile shear zone (Bassett et al. 1986). It is possible, but unproven, that this area originally formed a basement to the Monian Supergroup (cf. Barber & Max 1979). Gneisses in the Rosslare Complex of southeast Ireland may represent a continuation of this terrane, although here again they are isolated within major, steep mylonite zones. The age of the earliest amphibolite facies metamorphism in the Rosslare Gneisses is unknown, although late Precambrian (c. 626Ma) 4~ ages have been obtained from amphiboles interpreted as crystallized during a second metamorphic event (Max & Roddick 1989). The Anglesey blueschist belt includes ophiolitic epidote + sodic amphibole metabasites and phengitic metasediments, with lawsonite being known from the eastern margin (Gibbons & Mann 1983; Gibbons & Horfik 1989). a~ amphibole and phengite data indicate uplift ages of c. 550 Ma for the blueschist event, and c. 590 Ma for an earlier greenschist metamorphism (Dallmeyer & Gibbons 1987). Using textural and mineral chemical evidence Gibbons & Gyopari (1986) have argued that this greenschist metamorphism may record sub-sea floor metamorphism of oceanic crust. The blueschists are isolated by tectonic contracts: to the east they are thrust over Gwna m61ange, whereas to the west they are separated from the m61ange by a steep mylonitic schist belt (Berw shear zone) in which kinematic indicators record sinistral transcurrent movements (Gibbons 1987). The Berw shear zone is interpreted as recording early ductile movements along the Menai Strait fault system, a terrane boundary frequently reactivated during Phanerozoic time to produce a network of brittle faults. Another early, steep mylonite belt occurs on the Ll~n Peninsula and juxtaposes Gwna m61ange against a unit of mostly dioritic to granitic rocks known as the Sarn Complex (Gibbons 1983a). The calc-alkaline Sarn Complex has been interpreted as the northwestern margin of the Avalonian area in Britain and has yielded a poorly constrained Rb-Sr WR age of 549 + 19 Ma (Beckinsale et aL 1984). The docking of terranes along the Menai Strait fault system was presumably younger than the formation of the Sarn Complex and blueschists. A minimum docking age is provided by Arenig overstep across northeast Wales, and possibly by an apparent overstep on to Anglesey of Precambrian Arfon Group lithologies (Reedman et al. 1984; Tucker & Pharaoh 1991). In addition, Monian-type fragments are known from Lower and Middle Cambrian sediments on the Welsh mainland (Nicholas 1915; Greenly 1919, 1923; Gibbons 1983b), and detrital blue amphibole occurs within the Lower/Middle Cambrian St Non's Sandstone in southwest Wales (Mackay 1987). Present evidence therefore implicates a late Precambrian or early Cambrian age for the transcurrent faulting that produced the shear zones during terrane dispersal along the Avalonian margin.
Central England, the Welsh Borderland and South Wales
Most exposures southeast of the Menai Strait line record the intrusion and eruption .of intermediate to acid calc-alkaline igneous rocks and are interpreted as produced by subduction-related arc magmatism. Sedimentary basins developed upon and eroded from this igneous basement include much volcaniclastic material and represent interarc sequences, The largest British Avalonian inliers occur in Charnwood Forest (Charnian Supergroup), the Church Stretton and Wrekin areas in Shropshire (Longmyndian Supergroup and Uriconian Volcanic Group), and in the Malverns (Matvernian Complex). Similar rocks continue into southwest Wales and the Avalon Peninsula of southeast Newfoundland. The oldest Precambrian ages have been obtained from plutonic rocks in the Malverns (e.g. 681 + 53 Ma Rb-Sr WR; Beckinsale et aL 1981), at Stanner-Hanter along the Welsh border (702 + 8 Ma Rb-Sr WR; Patchett et al. 1980), and in the Johnston Igneous Complex of southwest Wales (643 + 5 Ma U-Pb zircon; Patchett & Jocelyn 1979). The Malvernian Complex is a north-south trending inlier composed mostly of diorite and tonalite, with some granite and small areas of ultramafic hornblendite (Lambert & Holland 1971). The whole complex is intruded by microdiorite dykes and affected by extensive ductile and brittle ~hearing. The igneous rocks exposed at Stanner-Hanter (Holgate & Hallowes 1941) lie along the southwestern continuation of the Church Stretton Fault and comprise gabbroic, microgabbroic and microgranitic rocks. The Johnston Complex is dominantly dioritic, but shows a mixture of tonalitic, granodioritic and granitic rocks comparable to the plutonic roots of modern volcanic arc systems (Thorpe et al. 1984). An exception to the dominantly igneous nature of the oldest rocks exposed in England is provided by the metamorphic lithologies known as the Rushton Schists and Primrose Hill Gneisses that crop out in Shropshire within the northeast extension of the Church Stretton Fault System. The Primrose Hill Gneisses are undated, and the age of the Rushton Schists is not well constrained, with the oldest radiometric age obtained being recorded as 667 + 20 Ma (Rb-Sr WR) (Thorpe et al. 1984). The Rushton Schists are garnet-muscovite-biotite, greenschist facies pelites. Little is known about these rocks, or the nearby, coarser grained, biotite-rich Primrose Hill Gneisses. They could represent old metamorphic basement to the volcanic arc, or they may be related to the reworking of Avalonian rocks by early ductile movements along the Welsh Borderland Fault System.
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
10
PRECAMBRIAN
Continuing intrusive calc-alkaline igneous activity is recorded by several inliers such as the St David's Granophyre in southwest Wales (Patchett & Jocelyn 1979), the older Charnian diorites of Central England (Cribb 1975; Pankhurst 1982) and the Ercall Granophyre in the Welsh Borderland (Patchett et al. 1980). Acid to intermediate volcanic rocks are also well represented, e.g. the Uriconian Volcanic G r o u p of the Welsh Borderland, the Warren House Volcanic Formation of the Malverns, and the Pebidian Volcanic G r o u p in southwest Wales. All these igneous rocks appear to be latest Precambrian to earliest Cambrian in age, although in some cases there are problems in reconciling the radiometric and biostratigraphical data (e.g. Cope & Gibbons 1987). The Longmyndian (Welsh Borderland) and Charnian (Central England) Supergroups provide the best record of sedimentary basins developed on or marginal to the arc (Pauley 1990). The 6.5 km Longmyndian succession records the filling of a turbidite basin by deltaic and finally alluvial sediments. The 3.5 km Charnian sequence contains more pyroclastic material and was deposited closer to volcanic activity than the Longmyndian (Moseley & F o r d 1985, 1989). Volcaniclastic sediments from Charnwood Forest, the Carmarthen area in southwest Wales, and in southeast Newfoundland (Conception Group) have all yielded a similar Ediacaran fauna of Vendian age (Ford 1980; Cope 1977; Misra 1969). These sediments were folded and variably cleaved prior to Lower Cambrian marine transgression. Deformation within the igneous basement prior to this transgression was restricted to brittle faults and mylonite zones, with good examples of the latter being exposed in the Malverns.
Channel Islands The Channel Islands belong geologically to the Armorican Massif of northwest France. They display an ancient ( > 2000 Ma) Icartian basement of migmatitic pelites, amphibolites and granitic augen gneisses that crops out as a narrow belt from Cap de la Hague through Guernsey and Sark to the northwest corner of Brittany (Calvez & Vidal 1978). Much younger, polydeformed, lower greenschist facies metasediments exposed on Jersey (Jersey Shale Formation) and Guernsey (Pleinmont Formation) have been correlated with cl 7 0 0 M a Brioverian sequences exposed on the French mainland (e.g. Helm 1983). The Icartian gneisses are intruded by sheets of foliated, calc-alkaline granitic rocks (mostly quartz-diorites), seen on Guernsey (Perelle quartzdiorite), Sark and Alderney (Power et al. 1989). These plutonic rocks appear to represent the earliest phase o f Cadomian, subduction-related magmatism in the Armorican area. Several attempts at producing radiometric ages on these rocks have failed to produce good results, with maximum ages falling into the range 690-570 M a (Rb-Sr W R & K - A r hornblende) but showing unacceptably large errors. Later Cadomian magmatism produced a suite of plutons widely exposed on Guernsey, Alderney and Jersey. These range from layered hornblende gabbros through appinitic diorites to granites, with mixed-magma textures being common. Cadomian volcanic rocks in the Channel Islands are restricted to exposures of the Jersey Volcanic G r o u p on Jersey. Rb-Sr whole rock ages on the younger Cadomian plutonic suite are in the range 550-426 M a (Strachan et al. 1989), the younger dates conflicting with a supposedly Cambrian age for an unconformably overlying clastic sequence on Alderney (Alderney Sandstone) and Jersey (Rozel Conglomerate Formation) (Sutton & Watson 1970; Went et al. 1988). These red beds have close lithological similarities with overstep Cambrian sequences on the French mainland. Sinistral, transcurrent shear zones dated as c. 540 Ma cut Brioverian rocks on mainland France (Strachan et al. 1989) and probably record the same terrane dispersal event that produced the Menai Strait line. Overall, the late Precambrian geology of the southern British Isles can be viewed as divisible into three superterranes (i.e. groups of terranes) that may be classified as Monian (Anglesey, Western Ll~,n, southeast Ireland), Avalonian (Sarn Complex of Lb)n, Central England, Welsh Borderland, southwest Wales), and Cadomian (Channel Islands). The calc-alkaline magmatism that dominates both Avalonian and Cadomian areas suggests that both areas were part of the same subduction-related arc system. Furthermore, a metamorphic source south of the British Avalon is suggested both by schistose detritus in Palaeozoic sediments (Cope & Bassett 1987) and foliated metagranitic xenoliths within Devonian lavas in Cornwall (Goode & Merriman 1987). However, p r o o f of close proximity between the British Avalonian and Cadomian areas during late Precambrian times does not exist. Phanerozoic fault movements, especially those in early Cambrian and Variscan times, are likely to have displaced these areas by an unknown amount. WG
R e f e r e n c e s
ALLEN'P., SUTTONJ. & WATSONJ. V. 1974. 'Torridonian' tourmaline-quartz pebbles and the Precambrian crust northwest of Britain. Journal of the Geological Society, London, 130, 85-91. ALLISON,A., MAY, e. • STRACHAN,R. A. 1989. Excursion guide to the Moine geology o/the Scottish Highlands. Scottish Academic Press, Edinburgh, 270. ANDERTON, R. 1976. Tidal shelf sedimentation: an example from the Scottish Dalradian. Sedimentology, 23, 429-458. -1980. Distinctive pebbles as indicators of Dalradian provenance. Scottish Journal of Geology, 16, 143-152.
BASSETT,M. G., BEVINS,R. E., GtBaONS,W. & HOWELLS,M. F. 1986. A geotraverse through the Caledonides of Wales. In: FETTES, D. J. & HARRIS, A. J. (eds) Synthesis of the Caledonian Rocks of Britain. Reidel, Dordrecht, 29-77. BARBER, A. J. & MAX, M. D. 1979. A new look at the Mona Complex (Anglesey, North Wales). Journal of the Geological Society, London, 136, 407-432. BECKINSALE,R. D., EVANS,J. A., THORPE, R. S., GmaONS, W. & HARMON,R. S. 1984. Rb-Sr whole-rock isochron ages, 6lsO values and geochemical data for the Sarn Igneous Complex and the Parwyd Gneisses of the Mona Complex of Ll~n, North Wales. Journal of the Geological Society, London, 141, 701-709. - - , THORPE, R. S., PANKHURST, R. J. & EVANS., J. A. 1981. Rb-Sr whole-rock isochron evidence for the age of the Malvern Hills Complex. Journal of the Geological Society, London, 138, 69-73. BLAKE, J. F. 1888. On the Monian System of rocks. Quarterly Journal o7 the Geological Society, London, 44, 463-546. BROOK, M., BREWER,M. S. & POWELL,D. 1976. Grenville age for rocks in the Moine of northwest Scotland. Nature, 260, 515. CALVEZ, J. Y. 8s VIDAL, P. 1978. TWO billion year old relicts in the Hercynian belt of Western Europe. Contributions to Mineralogy and Petrology, 65, 395-399. CLIFF, R. A. & REX, D. C. 1989. Evidence for a 'Grenville' event in the Lewisian of the northern Outer Hebrides. Journal of the Geological Society, London, 146, 921-924. COPE, J. C. W. 1977. An Ediacara type fauna from South Wales. Nature, 268, 624. - d~ BASSETT,M. G. 1987. Sediment sources and Palaeozoic history of the Bristol Channel area. Proceedings of the Geologists Association, 98, 315-330. - & GIBBONS,W. 1987. New evidence for the relative age of the Ercall Granophyre and its bearing on the Precambrian-Cambrian boundary in southern Britain. Geological Journal, 22, 53--60. CRIBB, S. J. 1975. Rb-Sr ages and strontium isotope ratios from the igneous rocks of Leicestershire. Journal of the Geological Society, London, 131, 203-212. DALLMEYER, R. D. & GIBBONS, W. 1987. The a g e of blueschist metamorphism in Anglesey, North Wales: evidence from *~ mineral dates of the Penmynydd schists. Journal of the Geological Society, London, 144, 843-852. ELLIOTT, D. & JOHNSON,M. R. W. 1980. Structural evolution in the northern part of the Moine thrust belt, NW Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 71, 6%96. FORD, T. D. 1980. The Ediacaran fossils of Charnwood Forest, Leicestershire. Proceedings of the Geologists' Association, 91, 81-84. GIBBONS, W. 1983a. The Monian Penmynydd Zone of Metamorphism in Ll~n, North Wales. Geological Journal, 18, 1-21. - 1983b. Stratigraphy, subduction and strike-slip faulting in the Mona Complex of North Wales - a review. Proceedings of the Geologists" Association, 94, 147163. - 1987. Menai Strait Fault System: An early Caledonian terrane boundary in North Wales. Geology, 15, 744-747. -1989. Suspect terrane definition in Anglesey, North Wales. In: DALLMEYER, R. D. Terranes in the Circum-Atlantic Orogen. Geological Society of America, Special Paper 230, 59-66 - - & GYOPARI, M. 1986. A greenschist protolith for blueschist in Anglesey, UK. In: EVANS,B. W. & BROWN, H. H. (eds). Blueschists and Eclogites. Geological Society of America Memoir 164, 217-228. - & HOR~,K,J. M. 1989. Contrasting metamorphine terranes in Northwest Wales. In: D'LEMOS, R. S., STRACHAN,R. A. & TOPLEY, C. G. (eds) The Cadomian Orogeny. Geological Society, London, Special Publication, 51, 31 5-327. - & MANN, A. 1983. Pre-Mesozoic lawsonite in Anglesey, North Wales - the preservation of ancient blueschists. Geology, 11, 3-6. GOODE, A. J. J. & MERRIMAN,R. J. 1987. Evidence of crystalline basement west of the Land's End Granite, Cornwall. Proceedings of the Geologists" Association, 98, 39-43. GREENLY, E. 1919. The Geology of Anglesey. Memoir of the Geological Survey of Great Britain (2 vols). HMSO, London. - 1923. The succession and metamorphism in the Mona Complex. Quarterly Journal of the Geological Society of London, 79, 334-351. HARRIS, A. L., BALDWIN,C. T., BRADBURY,H. J., JOHNSON, H. D. & SMITH, R. A., 1978. Ensialic basin sedimentation; the Dalradian Supergroup. In: BowF,s, D. R. & LEAKE, B. E. (eds) Crustal evolution in northwestern Britain and adjacent regions. - & JOHNSON, M. R. W. 1991. Moine. In: CRAIG, G. Y. (ed.) Geology of Scotland. Geological Society, London, 3rd edition, 87-124. HASELOCK,P. J. & GmaONS, W. 1990. The Central Highlands Controversy: a traverse through the Precambrian metasediments of the Central Highlands of Scotland. Episodes, 13, 113-115. HELM, D. G. 1983. The structure and tectonic evolution of the Jersey Shale Formation, St. Ouen's Bay, Jersey, Channel Islands. Proceedings of the Geologists' Association, 94, 201-216. HICKMAN,A. H. 1975. The stratigraphy of late Precambrian metasediments between Glen Roy and Lismore. Scottish Journal of Geology, 11, 117-142. HOLDSWORTrl, R. E. 1987. Basement-Cover relationships, reworking and Caledonian ductile thrust tectonics of the northern Moine, N W Scotland. PhD Thesis, University of Leeds. HOLGATE, N. & HALLOWES,K. A. 1941. The igneous rocks of the Stanner-Hanter District, Radnorshire. Geological Magazine, 78, 241-267. JOHNSTONE, G. S. 1975. The Moine Succession. In: H^RRIS, A.L. (ed.) d Correlation of Precambrian Rocks in the British Isles. Geological Society, London, Special Report 6, 30-42. --, SMITH, D. I. 8s HARRIS,A. L. 1969. Tbe Moinian Assemblage of Scotland. In: KAY, M. (ed.) North Atlantic Geology and Continental Drift. American Association of Petroleum Geologists Memoir, 12, 15%180. LAILEY, M., STEIN, A. M. & RESTON, T. J. 1989. The Outer Hebrides fault: a major Proterozoic structure in NW Britain. Journal of the Geological Society, London, 146, 253-259. LAMBERT, R. St. J. & HOLLAND,J. G. 1971. The petrography and chemistry of the Igneous Complex of the Malvern Hills, England. Proceedings of the Geologists' Association, 8 2 , 323-35 I. LAWSON, D. E. 1965. Lithofacies and correlation within the lower 'Torridonian'. Nature, 207, 706-708. -1970. The 'Torridonian' rocks of the Gairloch area and parts oJ Wester Ross, Sutherland, N W Scotland. PhD thesis, University of Reading. -1972. 'Torridonian' volcanic sediments. Scottish Journal of Geology, 8(4), 345362. MACKAY, A. W. 1987. The sedimentology and petrology of the Cambrian sediments below the Lingula Flags on the St David's Peninsula, Southwest Wales. MSc thesis, University of Wales (Cardiff). McQUILLtN, R. & BtNNS, P. E. 1973. Geological structure in the Sea of the Hebrides. Nature, 241, 2-4.
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CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
12
PRECAMBRIAN
MAX, M. D. & RODDICK, J. C. 1989. Age of Metamorphism of the Rosslare Complex, SE Ireland. Proceedings of the Geologists' Association, 100, I 13-122. MISRA, S. B. 1969. Late Precambrian (?) fossils from Southeast Newfoundland. Geological Society of America Bulletin, 80, 2133-2140. MOORBATH, S. 1969. Evidence for the age of deposition of the 'Torridonian' sediments of NW Scotland. Scottish Journal of Geology, 5(2), 154-170. MOSELEu J. & FORD, T. D. 1985. A stratigraphic revision of the Late Precambrian rocks of Charnwood Forest, Leicestershire. Mercian Geologist, 10, 1-18. -& -1989. The sedimentology of the Charnian Supergroup. Mercian
ROGERS, G., DEMPS'rER, T. J., BLUCK, B. J. & TANNER, P. W. G. 1989. A high precision U-Pb age for the Ben Vuirich granite: implications for the evolution of the Scottish Dalradian Supergroup. Journal of the Geological Society, London, 146, 789-798. --, KROGH, T. E., BLUCK, B. J. & KWOK, Y. Y. 1989. Sedimentary provenance ages of the "Torridonian' sandstone using single grain U-Pb zircon analysis.
Abstracts, Developments in Sedimentary Provenance Studies, BSRG & Petroleum Group Joint Meeting, Geological SocieO' of London. SANDERS, I. S. & JOHNSTON, J. D. 1989. The 'Torridonian' Stac Fada Member: an extrusion of fluidised peperite? Transactions of the Royal Society of Edinburgh:
Geologist. MUIR, M. D., BLISS, G. M., GRANT, P. R. & FISCHER, M. 1979. Palaeontological evidence for the age of some supposedly Precambrian rocks in Anglesey, North Wales. Journal of the Geological Society, London, 136, 61-64. NICHOLAS, T. C. 1915. The Geology of the St Tudwal's Peninsula. Quarterly Journal of the Geological Society, London, 71, 83-143. NICHOLSON, P. G. 1989. Structure, geometry and genesis of fluvial sediment sheets and macroforms from the Precambrian 'Torridonian' Applecross Formation, Handa Island, NW Scotland. Programme and Abstracts, Fourth International
Conference on Fluvial Sedimentology, Sitges, Spain. PANKHVRST, R. J. 1982. Geochronological tables for British igneous rocks. Appendix C. In: SUTHERLAND, D. S. (ed.) Igneous rocks of the British Isles. Wiley, Chichester, 575-582. PATCHETT, P. J., BVLUND, G. & UPTON, B. G. J. 1978. Palaeomagnetism and the Grenville orogeny: New Rb-Sr ages from dolerites in Canada and Greenland. Earth and Planetary Science Letters, 40, 349-364. - - , GALE, N. H., GOODWIN, R. & HUMM, M. J. 1980. Rb-Sr whole-rock isochron ages of late Precambrian to Cambrian igneous rocks from Southern Britain. Journal of the Geological Society, London, 137, 649-656. -& JOCELVN, J. 1979. U-Pb zircon ages for late Precambrian igneous rocks in South Wales. Journal of the Geological Society, London, 137, 649-656. PAULEY, J. C. 1989. The Longmyndian Supergroup and related Precambrian sediment of England and Wales. In: STRACHAN, R. A. & TAYLOR, G. K. (eds) Avalonian and Cadomian Geology of the North Atlantic. Blackie, Glasgow, 5-27. PEACH, B. N. & HORNE, J. 1910. The geology of Glenelg, Lochalsh & south-east part of Skye. Memoir of the Geological Survey of Great Britain. PEAT, C. J. 1984. Comments on some of Britain's oldest microfossils Journal of Micropalaeontology, 3, 65-7l. PIASECKI, M. A. J. 1980. New light on the Moine rocks of the Central Highlands of Scotland. Journal of the Geological Society, London, 137, 41-59. -t~ VAN BREEMAN, O. 1983. Field and isotopic evidence for a c. 750Ma tectonothermal event in the Moine rocks in the Central Highland region of the Scottish Caledonides. Transactions of the Royal Society of Edinburgh: Earth Sciences, 73, 119-134. - - - n g- & l a E n d WRIGHT, A. D. 1981. The Late Precambrian geology of Scotland, and Wales In: KERR, J. & FERGUSON,A. J. (eds) Geology of the North Atlantic Borderlands. Memoir of the Canadian Society of Petroleum Geologists, 7, 57-94. PIPER, J. O. A. 1983. The Precambrian palaeomagnetic record: the case for the Proterozoic Supercontinent. Earth and Planetary Science Letters, 59, 61-89. POWER, G. M., BREWER,T. S., BROWN, M. & GmBONS, W. 1989. Late Precambrian foliated plutonic complexes of the Channel Islands and La Hague - Early Cadomian Plutonism. In: D'LEMOS R. S., STRACHAN,R. A. t~ TOPLEY, C. G. (eds) The Cadomian Orogeny. Geological Society, London, Special Publication, 51,215-230. RAMSEY, J. G. 1958. Moine-Lewisian relations at Glenelg, Inverness-shire. Quarterly Journal of the Geological Society, London, 113, 487-523. RATHBONE, P. A. &. HARRIS, A. L. 1979. Basement-Cover relationships at Lewisian inliers in the Moine rocks. In: HARRIS, A. L., HOLLAND, C. H. d~ LEAKE, B. E. (eds) The Caledonides of the British Isles- Reviewed. Geological Society, London, Special Publication, 8, 101-109. REEDMAN, A. J., LEVERIDGE, B. E. & EVANS, R. B. 1984. The Arfon Group ("Arvonian") of North Wales. Proceedings of the Geologists' Association, 95, 313-322. ROBERTS, A. M. & HARRIS, A. L. 1983. The Loch Quoich L i n e - a limit of early Palaeozoic crustal reworking in the Moine of the Northern Highlands of Scotland. Journal of the Geological Society, London, 140, 883-892. --, SMITH, D. I. & HARRIS, A. L. 1984. The structural setting and tectonic significance of the Glen Dessary Syenite, Inverness-shire. Journal of the Geological Society, London, 141, 1033-1042. - - , STRACHAN,R. A., HARRIS, A. L., BARR, D. & HOLDSWORTH, R. E. 1987. The Sgurr Beag nappe: a reassessment of the stratigraphy and structure of the Northern Highland Moine. Bulletin of the Geological Society of America, 98, 497-506.
Earth Sciences, 80, 1-4. --,
VAN CALSTEREN,P. W. C. • HAWKESWORTH,C. J. 1984. A Grenville Sm-Nd age for the Glenelg eclogite in northwest Scotland, Nature, 312, 439-440. SHACKLETON,R. M. 1975. Precambrian rocks of North Wales. In: HARRIS,A. L. et al. (eds) A correlation of the Precambrian rocks in the British Isles. Geological Society, London, Special Report, 6, 76-82. SOPER, N. J. &. ANDERTON, R. 1984. Did the Dalradian slides originate as extensional faults? Nature, 307, 357-360. STEIN, A. M. 1988. Basement controls upon Hebridean basin development. Basin Research, 1(2), 107-119. STEWART, A. D. 1969. 'Torridonian' rocks of Scotland reviewed. In: KAY, M. (ed.) North Atlantic - Geology and Continental Drift. Memoir 12, 595-608. 1972. Precambrian landscapes in NW Scotland. Geological Journal 8, I i 1-124. -1978. Itinerary II: Stoer and Loch Assynt ('Torridonian'). In: BARBER,A. J., BEACH, A., PARK, R. G., TAMEY, J. & STEWART, A. D. The Lewisian and 'Torridonian' rocks of N W Scotland. The Geologists" Association Guide, 21, 2735. 1982. Late Proterozoic rifting in NW Scotland: the genesis of the 'Torridonian'. Journal of the Geological Society, London, 139, 413-420. -1988a. The Stoer Group, Scotland. In: WINCHESTER,J. A. (ed.) Later Proterozoic Stratigraphy of the Northern Atlantic Regions. Blackie, Glasgow, 97-103. -1988b. The Slept and Torridon Groups. In: WINCHESTER, J. A. (ed.) Later Proterozoic Stratigraphy of the Northern Atlantic Regions. Blackie, Glasgow, 104-112. STRACHAN, R. A., TRELOAR, P. J., BROWN, M. ,ft D'LEMoS, R. S. 1989. Cadomian terrane tectonics and magmatism in the Armorican Massif. Journal of the Geological Society, London, 146, 423-426. SUTTON, J. & WATSON, J. V. 1964. Some aspects of 'Torridonian' stratigraphy in Skye. Proceedings of the Geologists" Association, 75, 251-289. & -1970. The Alderney Sandstone in relation to the ending of plutonism in the Channel Islands. Proceedings of the Geologists" Association, 81, 577-584. THORPE, R. S., BECKINSALE,R. D., PATCHETT, P. J., PIPER, J. D. A., DAVIES, G. R. & EVANS, J. A. 1984. Crustal growth and Late Precambrian-Early Palaeozoic plate tectonic evolution of England and Wales. TIETZSCH-TVLER,D. & PHILLIPS, E. 1989. Correlation of the Monian Supergroup in NW Anglesey and the Cahore Group is SE Ireland. Journal of the Geological Society, London, 146, 417-418. TORSVIK, T. H. & STURT, B. A. 1987. On the origin and stability of remanence and the magnetic fabric of the 'Torridonian' red beds, NW Scotland. Scottish Journal of Geology, 23(1 ), 23-38. TUCKER, R. D. & PHARAOH, T. C. 1991. U-Pb zircon ages for Late Precambrian igneous rocks in Southern Britain. Journal of the Geological Society, London, 148, 435-444. UPEOLD, R. L. 1984a. Sedimentology of the Poll a' Mhuilt Member, Stoer Group, N W Scotland. PhD thesis, University of Reading. 1984b. Tufted microbial (cyanobacterial) mats from the Proterozoic Stoer Group, Scotland. Geological Magazine, 121(4), 351-355. WENT, D. J., ANDREWS,M. J. & WILLIAMS,B. P. J. 1988. Processes of alluvial fan sedimentation, basal Rozel Conglomerate Formation, La Tete des Hougues, Jersey, Channel Islands. Geological Journal, 23, 75-84. WILLIAMS, G. E. 1966. Palaeogeography of the 'Torridonian' Applecross group. Nature, 209, 1303-1306. -1968. 'Torridonian' weathering, and its bearing on 'Torridonian' palaeoclimate and source. Scottish Journal of Geology, 4(2), 164-187. 1969a. Characteristics and origin of a Precambrian pediment. Journal of Geology, 77, 183-207. 1969b. Petrography and origin of pebbles from 'Torridonian' strata (Late Precambrian), NW Scotland. In: KAY, M. (ed.) North Atlantic - Geology and Continental Drift. American Association of Petroleum Geologists Memoir, 12, 609-629. WOOD, M. & NICHOLLS, G. D. 1973. Precambrian stromatolitic limestones from Northern Anglesey. Nature, 241, 65. -
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Cambrian M . D. B R A S I E R ,
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Aspects of the history of the Cambrian System, and especially its usage in the British Isles, are discussed by Holland (1974). Rushton (1974), Cowie (1974) and Dhonau & Holland (1974) give general accounts of Cambrian stratigraphy in the British Isles, with extensive bibliographies, and the correlation is discussed by Cowie et al. (1972). The limits of the Cambrian System are still unsettled. The Precambrian/ Cambrian boundary has been the subject of much recent research, concentrated especially on strata of Tommotian and Meishucunian age (Cowie & Brasier 1989). These stages are characterized by faunas of small shelly fossils but biostratigraphical correlation based on them is still rather insecure and a consensus on the basal Cambrian stratotype has yet to be reached (Brasier 1989b). The upper limit of the Cambrian (the base of the Ordovician System) is likewise unsettled. Cowie et al. (1972) took the base of the Ordovician at the base of the Arenig Series, thereby including the Tremadoc Series in the Cambrian. Nowadays a level at the base of the Tremadoc, for long adopted in continental Europe and elsewhere, is more widely accepted in Britain, and is employed here. However, the exact horizon for the base of the Ordovician, and the locality at which it is to be defined, are still under debate. There are no standard series defined in the Cambrian System. It has long been customary to speak of Lower, Middle and Upper Cambrian, and treat these as series, but they are employed in different senses in different parts of the world. To avoid that ambiguity Cowie et al. (1972) applied the series names Comley, St David's and Merioneth to the Lower, Middle and Upper Cambrian of British usage, detailing how they were to be recognized but without providing formal definitions. Despite their unsatisfactory features these series are adopted here and provide ready cross-reference to Cowie et al.'s correlation charts. Measurement of isotopic ages on Cambrian rocks has given very discordant results, as discussed by Cowie & Johnson (1985) and Odin et al. (1985). For many years the (undefined) base of the Cambrian was put at about 570 Ma. Results from the earliest Cambrian in southern China have given markedly earlier dates of around 600Ma, whereas, recent results from Morocco, France and England have indicated much younger dates for latest Precambrian-early Cambrian events of around 530Ma (e.g. Cope & Gibbons 1987). This discrepancy appears too great to be accounted for by uncertainties in the definition of the base of the Cambrian or uncertainties in faunal correlation and is at present unresolved. The ages given on the maps are therefore notional and subject to revision. Palaeogeographically it is accepted that northern Britain, along with other microcontinental plates, lay in low latitudes adjacent to the Laurentian continent. The position of the southern part of Britain is less certain. There is good evidence for a close connection with the Avalon area of southeast Newfoundland where the Cambrian lithological and faunal successions are particularly similar to those of central England, but the exact make-up of the Avalonian plate is uncertain and it is not yet clear how close it lay to the proto-Gondwanan continent, though it seems clear that it was separated from Baltica (Conway Morris & Rushton 1988); there is some indication that Avalonia lay at mid-latitudes at the beginning of the Cambrian Period and had migrated to higher latitudes by the end of the Period (e.g. Theokritoff 1979; Thorpe et al. 1984). AWAR
s
Southern British Isles: Comley
After widespread igneous activity and the Cadomian orogenic phase of about 550 Ma BP, much of the southern British Isles was broken into positive blocks and sinking basins. The outermost of these was the Leinster marginal basin of the so-called Leinster terrane, in which were deposited fanglomerates, thick turbidites and deep water shales of the Cambrian Bray Series with Oldhamia; shoaling and a northwest source are suspected in the Howth area (e.g. Crimes 1976) while a southeast source in the Wexford and Wicklow regions is arguably an Irish Sea Horst, composed of Archaean to Vendian metamorphic rocks that stretched from southeast Ireland to the Mona Complex of Ll~n and Anglesey. Max et al. (1990) have proposed that the Leinster region is an assemblage of several discrete Lower Palaeozoic terranes. The existence of an Irish Sea Horst in Cambrian times has been debated since no direct contact is seen between the Mona Complex and Cambrian strata. Indeed, several lines of evidence have been put forward to suggest the Mona Complex was deposited during the Cambrian in part: supposed Lower Cambrian microfossils in the Gwna Group near the top (Muir et al. 9 1992The GeologicalSociety
& A. W . A. R U S H T O N
1979); dubious trace fossils in the South Stack Group near the base (op. cit.); and comparisons with the Cambrian Cahore Group of southeast Ireland (Tietzsch-Tyler & Phillips 1989). But the microfossils are poorly preserved and of questionable stratigraphical value while isotopic dates of c. 542562 Ma (Thorpe et al. 1984) fit better with a latest Precambrian age for the final phase of metamorphism and intrusion in the Mona Complex. The tropical carbonates of the Gwna Group are also inconsistent with the generally cool water character of the Avalon terrane inferred by Conway Morris & Rushton (1988) and suggest an exotic origin for this suite. To the southeast of the Irish Sea Horst lay the Welsh Basin and the Midland Platform, both floored by latest Precambrian-earliest Cambrian volcanics (e.g. Patchett et al. 1980; Thorpe et al. 1984). Strata in this region can be correlated using an Avalonian biostratigraphical scheme, being developed in southeast Newfoundland and England (Bengtson & Fletcher 1983; Shergold & Brasier 1986; Brasier 1989a), broadly as follows: Protolenus Zone, Strenuella Zone, Callavia Zone, Rhombocorniculum insolutum/Camenella baltica Zone, Aldanella attleborensis Zone.
Rapid deepening in the Welsh Basin led to a change from molasse and shoreline (about the attleborensis Zone) to subsequent deeper marine shales and turbidite deposits in North Wales. The proximal Rhinog Grits in the Harlech Dome and St Tudwal's Peninsula generally had axial, NE-SW currents while more distal silts and muds accumulated in the Caernarvon Slate Belt to the north. A regressive episode, with thin Mn-bearing shales, arguably marks the end of this depositional sequence in earliest Middle Cambrian times, prior to uplift and erosion of the basin margins. Fauna, facies and sequence stratigraphy indicate close relationships with Avalon terrane strata in England and maritime Canada. The Lower Cambrian in the St David's-Hayscastle area, South Wales, overlies late Proterozoic volcanic and plutonic rocks, with a deepening cycle from beach and coastal clastics of uncertain age to offshore shales with tuff bands, Teichichnus burrows and scarce Lingulella sp. (Caerfai Bay Shales) of presumed later early Cambrian age. These were followed by the shallower Caerbwdy Sandstone. Cambrian strata are missing south of the Johnston Thrust (Powell 1989) and the foregoing may have fringed a southerly landmass at this time. On the eastern margin of the Welsh Basin, latest Proterozoic rocks of the Uriconian and Longmyndian appear to have been folded and uplifted close to the Precambrian~Cambrian boundary, culminating in intrusion of the Ercall granophyre at 560 + 1 Ma (Tucker & Pharaoh 1991). Transgressions then laid down the fossiliferous strata of the Midlands in several distinct sequences with contrasting histories (Comley-Wrekin, Malvern, Banbury, Nuneaton-Charnwood). These fragments can best be understood through comparison with blocks and basins of the Burin, Bonavista and Avalon Peninsulas, southeast Newfoundland (e.g. Bengtson & Fletcher 1983; Brasier 1989a) in which three depositional cycles are often accompanied by alternating patterns of subsidence, ending locally with emergence. In England, cycle 1 contains abundant trace fossils and correlates with the Chapel Island-Random cycle of Newfoundland of early attleborensis age. Cycle 2 spans the upper attleborensis to insolutum/baltica Zones, indicating correlation with the Bonavista-lower Smith Point Limestone cycle of southeast Newfoundland. Cycle 3 brought with it the first trilobites in the region, and a sequence of faunas in the Callavia, Strenuella and Protolenus Zones. Correlation is made with the upper Smith Point Limestone and Brigus Formation shales of southeast Newfoundland (Brasier 1989a,b). Progressive submergence of a low-lying basement in the Comley-Wrekin area of Shropshire led to the succession of Wrekin Quartzite (cycle 1, thin), glauconitic Lower Comley Sandstone (cycle 2, thick), condensed, phosphatic Comley Limestone (cycle 3, thin), with emergence at the end of the early Cambrian. The sequence along the flanks of the Malverns appears to be similar but more thickly developed; cycle 3 is absent and may have been removed by subsequent uplift and emergence. The northern margin of the Midlands comprised a positive ridge of N W SE-trending Charnian volcanics, perhaps facing a North Sea-Central European Ocean (e.g. Wills 1978). In places these may have formed a land area in Cambrian times because Stockingford Shales cannot be traced over Charnwood Forest though they continue in subcrop to the northeast and from Nuneaton to the Derbyshire Dome region (Whitcombe & Maguire 1981a,b; Maguire 1987). Putative Lower Cambrian quartzites occur in boreholes in the northeast Midlands and may have been deposited on this Charnian i3
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
16 C2b:
CAMBRIAN Northern
British
Isles: Comley
Before the Cambrian Period, the region westward from Durness to Skye formed part of a land area that was probably continuous from Western Newfoundland to Greenland. Exposed Archaean and Proterozoic rocks provided the source of most clastic sediments of late Proterozoic and Cambrian age (e.g Swett & Smit 1972; Anderton 1982). Eustatic rises in sea level in the early Cambrian then led to a cover of clastic or carbonate sediments of North American type over a peneplained surface (e.g. Swett & Smit 1972). Rocks of the Lower Cambrian or Comley Series record the initial stages of this transgression. They can be subdivided according to the North American trilobite scheme or the Baltic acritarch/trilobite scheme, from the base upwards (Table 1). Table 1. Comparison of North American trilobite scheme and Baltic acritarch/trilobite scheme. North American
Baltic
Bonnia-Oleneltus Nevadella Fallotaspis non-trilobite
Vergale or Holmia B Vergale or Holmia B Talsy or Holmia A Lontova or sub-Holmia
Shoreline conglomerates and tidal quartzites with Skolithos burrows (the Eriboll Quartzite, e.g. Swett et al. 1971) have yielded non-diagnostic acritarchs but the unit may be of Talsy age (Downie 1982) or even of Lontova age, as in Spitsbergen (Knoll & Swett 1987). Similarities with Spitsbergen suggest that a widespread hiatus roughly equivalent to the Talsy Stage could lie between the Eriboll Quartzite and the Fucoid Beds. These deposits of the inner detrital belt were presumably displaced northwestwards by continuing transgression, so that by Bonnia-Olenellus/Vergale times the shoreline may have lain in the Rockall or Greenland region, and a widespread middle carbonate belt was developed over northwest Scotland with dolomitic mudstones yielding Olenellus (the Fucoid Beds). A brief regressive interlude of cross-bedded sandstones with Salterella is taken to mark the middle of the Bonnia-Olenellus zone here and allows correlation with the beginning of Grand Cycle C across the North American craton (Fritz & Yochelson 1988). This was followed by mainly subtidal dolomitized limestones with bioturbation and Salterella (the Grudaidh Formation) and by barren peritidal sediments of the Eilean Dubh Formation. Facies and stratigraphical position indicate carbonate sedimentation persisted in this region, with episodes of emergence and karstic erosion, until early Ordovician times (e.g. Palmer et al. 1980). An inferred break between the Eilean Dubh Formation (early to basal Middle Cambrian) and the Sailmhor Formation of early Ordovician age (S. Wright, unpublished) has been related to upwarping of the shelf, but this break may be much shorter than formerly thought. The succession in adjacent west Newfoundland preserves more facies belts (Knight & Boyce 1987) but shows an essentially similar history, from coastal clastics (Bradore Formation) through offshore muds and carbonates (Forteau Formation with Olenellus and archaeocyathan reefs) to siliciclastics with Salterella (Hawke Bay Formation, lower part) and stromatolitic carbonates (upper part). The latter span the circum-lapetus regression termed the 'Hawke Bay Event'. The area between the Moine Thrust and the Great Glen Fault has been partly overthrust onto the Northwest platform and must originally have lain further southeast. Dalradian and Cambrian outcrops are lacking but geophysical and tectonic evidence suggest that the margin of the Durness Platform lay between 30 and 120 km ESE of the thrust zone (Hutton et al. 1980; Butler 1982). It is now concluded that there are no strata of Cambrian age in the Dalradian tract of the Central Highlands (Grampian) terrane, as was previously believed (see Terranes section). In Early Cambrian times the whole belt was probably undergoing deep crustal thickening and metamorphism. Whether there was any subaerial expression is not known. Until recently, the Leny Limestone of the Callander area in the Highland boundary zone has been regarded as lying at or near the top of the Southern Highland Group of the Dalradian (Harris 1969). These dark, thin-bedded micritic limestones and black shales contain a fauna of pelagic eodiscids (Pagetides spp.) and benthic trilobites that compare with late Lower Cambrian slope facies of New York (e.g. Theokritoff 1981). It is considered here that the Leny succession is probably an integral part of the largely Ordoviclan Highland Border Complex and that its relationship to local Dalradian sheared grits and phyllites is one of complex underthrusting--the true Highland Boundary Fault. This line is in all probability a major terrane suture (Bluck 1983, 1985) and the Highland Border Complex forms a part of the Midland Valley terrane which did not finally suture with the Central Highlands Dalradian terrane until Devonian times (see Curry et al. 1982; Bluck et al. 1984; Ingham et al. 1986). A recently published early-mid Cambrian date for the Bute amphibolite (part of the Highland Border Complex) has been interpreted as an ocean current obduction event (Dempster & Bluck 1991). It is postulated that this margin to the Midland Valley (Laurentian) terrane was destructive at this, in contrast to the passive margin to the Hebridean cratonic terrane. A more rigid block, or blocks, further to the southeast of the Highland Boundary Fault in Cambrian times has been suggested from several lines of
evidence, e.g. southerly derivation of Southern Highland Group grits (not necessarily from the Midland Valley terrane), undeformed granitic and shallow water sedimentary c[asts transported southwards to Girvan and the Southern Uplands in the Ordovician (Longman et al. 1979; Rushton & Tripp 1979; Curry et al. 1982; Bluck et al. 1984; Ingham et al. 1986) and geophysical evidence for continental crust below the Midland Valley and Southern Uplands. The shelf, slope and basinal facies peripheral to the 'northern' side of the Midland Valley terrarte may be partly concealed beneath the southern part of the Central Highlands terrane: they are at least partly caught up in the Highland Border Complex which is itself largely concealed beneath the Upper Palaeozoic cover of the northern part of the Midland Valley. The block now referred to as the Midland Valley terrane has even been suggested as a link between the North American and Baltic cratons in Vendian times with the [apetus Ocean opening along its southeast margin in the Precambrian-Cambrian boundary interval (e.g. Anderton 1982), but there is no hard evidence for either supposition. New dates of between 560-530 Ma for andesitic and rhyolitic clasts from the northern belt of the Southern Uplands imply an arc source to part of the Southern Uplands at this time (Dempster & Bluck 1991). The palaeogeographical configuration is currently obscure. Rocks with a supposed Cambrian sponge from supposed Dalradian rocks in south Mayo (western Ireland) (Rushton & Phillips 1973) contain early Ordovician conodonts and they are now regarded as belonging to the Irish continuation of the Highland Border Complex of the Clew Bay line (Harper et al. 1989). The most striking evidence for an Iapetus Ocean in early Cambrian times lies in biofacies and lithofacies contrasts between the Scottish and AngloWelsh areas. In the latter region, the North American Laurentian fauna (e.g. Olenellus) and Cordilleran lithofacies (e.g. thick tropical carbonates) referred to above are absent; Avalonian faunas (e.g. Callavia, Protolenidae, Ellipsocephalidae) and cooler water lithofacies are characteristic, including Mn and P-rich shales and condensed nodular limestones. MDB, JKI
References
ALLEN, P. M., JACKSON,A. A. & RUSHTON,A. W. A. 1981. The stratigraphy of the Mawddach Group in the Cambrian succession of North Wales. Proceedings of the Yorkshire Geological Society, 43, 295-329. ANDERTON, R. 1982. Dalradian deposition and the late Precambrian~Cambrian history of the North Atlantic region: a review of the early evolution of the Iapetus Ocean. Journal of the Geological Society, London, 139, 421-431. BENGISON,S. & FLETCHER,T. P. 1983. The oldest sequence of skeletal fossils in the lower Cambrian of southeastern Newfoundland. Canadian Journal of Earth Sciences, 20, 525-536. BLOCK, B. J. 1983. Rtle of the Midland Valley of Scotland in the Caledonian Orogeny. Transactions of the Royal Society of Edinburgh: Earth Sciences, 74, 119-136. -1985. The Scottish paratectonic Caledonides. Scottish Journal of Geology, 21, 437-464. - - , INGHAM,J. K., CURRY, G. B. & WILLIAMS,A. 1984. VI. Stratigraphy and tectonic setting of the Highland Border Complex. In: CURRY, G. B., BLOCK, B. J., BURTON,C. J., INGHAM,J. K. SIVETER,D. J. & WILLIAMS,A. (eds) Age, evolution and tectonic history of the Highland Border Complex, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 75, 113-133. BRASmR, M. D. 1986. The succession of small shelly fossils (especially conoidal microfossils) from English Precambrian-Cambrian boundary beds. Geological Magazine, 123, 237-256. -1989a. Sections in England and their correlation. In COWIE,M. D. & BRASIER, M. D. (eds) The Precambrian-Cambrian Boundary. Clarendon, Oxford, 82-104. -1989b. Towards a biostratigraphy of the earliest skeletal biotas. In Cowm, M. D. & BRASmR,M. D. (eds) The Precambrian-Cambrian Boundary. Clarendon, Oxford, 117-165. -& HEWI'r3, R. A. 1981. Faunal sequence within the Lower Cambrian "Nontrilobite" Zone (s.1.) of central England and correlated regions. United States Geological Survey, Open File Report 81-743, 26-28. BUTLER,R. W. H. 1982. A structural analysis of the Moine thrust zone between Loch Eriboll and Foinahaven, north-west Scotland. Journal of Structural Geology, 4, 19-29. CO,BOLD,E. S. 1927. The stratigraphy and geological structure of the Cambrian area of Comley (Shropshire). Quarterly Journal of the Geological Society, London, 83, 551-573. CONWAYMoRRiS, S. & RUSHTON,A. W. A. 1988. Precambrian to Tremadoc biotas in the Caledonides. In: HARalS, A. L. & FETrES, D. J. (eds) The CaledonianAppalachian Orogen. Geological Society, London, Special Publication, 38, 93109. COPE, J. C. W. & BASSET-r,M. G. 1987. Sediment sources and Palaeozoic history of the Bristol Channel area. Proceedings of the Geologists' Association. 98, 315-330. - - & GmBONS, W. 1987. New evidence for the relative age of the Ercall Granophyre and its beating on the Precambrian-Cambrian boundary in southern Britain. Geological Journal, 22, 53-60. COWIE,J. W. 1974. The Cambrian of Spitsbergen and Scotland. In: HOLLAND,C. H. (ed.) Cambrian of the British Isles, Norden and Spitsbergen (Lower Palaeozoic rocks of the World, 2), 123-155. Wiley, London. - - & BRASIER,M. D. (eds) 1989. The Precambrian-Cambrian Boundary. Clarendon, Oxford. -& JOHNSON,M. R. W. 1985. Late Precambrian and Cambrian geological time scale. In: SNELLING,N. J. (ed.) The Chronology of the geological record. Geological Society, London, Memoir, 10, 47~4. - - , ROSHTON,A. W. A. & STUBBLEVIELD,C. J. 1972. A correlation of Cambrian rocks in the British Isles. Geological Society, London, Special Report, 2. CRIMES, T. P. 1970. A facies analysis of the Cambrian of Wales. Palaeogeography, Palaeoclimatology and Palaeoecology, 7, 113-170. -1976. Trace fossils from the Bray Group (Cambrian) at Howth, Co. Dublin. Bulletin of the Geological Survey of Ireland, 2, 53~57.
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
l8
CAMBRIAN
& CROSSLEY, J. D. 1968. The stratigraphy, sedimentology, ichnology and structure of theLower Palaeozoic rocks of part of north-eastern Co. Wexford. Proceedings of the Royal Irish Academy, 67, Section B, ! 85-215. CURRY, G. B., INGHAM,J. K., BLUCK,B. J. & WILLIAMS,A. 1982. The significance of a reliable Ordovician age for some Highland Border rocks in Central Scotland. Journal of the geological Society, London, 139, 451-454. DEMPSTER,T. J. & BLUCK, B. J. 1991. The age and tectonic significance of the Bute amphibolite, Highland Border Complex, Scotland. Geological Magazine, 128, 77-80. DHONAU,N. B. & HOLLAND,C. H. 1974. The Cambrian of Ireland. In: HOLLAND,C. H. (ed.) Cambrian of the British Isles, Norden and Spitsbergen (Lower Palaeozoic rocks of the World, 2), 157-176. Wiley, London. DOWNIE, C. 1982. Lower Cambrian acritarchs from Scotland, Norway, Greenland and Canada. Transactions of the Royal Society of Edinburgh. Earth Sciences, 72, 257-285. FRITZ, W. H. & YOCHELSON, E. L. 1988. The status of Salterella as a Lower Cambrian index fossil. Canadian Journal of Earth Sciences, 25, 403-416. HARPER, D. A. Z., WILLIAMS,D M. & ARMSTRONG,H. A. 1989. Stratigraphical correlations adjacent to the Highland Boundary fault in the west of Ireland. Journal of the Geological Society, London, 146, 381-384. HARRIS, A. L. 1969. The relationship of the Leny Limestone to the Dalradian. Scottish Journal of Geology, 5, 187-190. HOLLAND,C. n . (ed.) 1974. Cambrian of the British Isles, Norden and Spitsbergen (Lower Palaeozoic rocks of the World, 2). Wiley, London. HUTTON,V. R. S., INGHAM,M. R. & MBIPOM,E. W. 1980. An electrical model for the crust and upper mantle in Scotland. Nature, 287, 30-33. INGHAM, J. K., CURRY, G. B. & WILLIAMS,A. 1986. Early Ordovician Dounans Limestone fauna, Highland Border Complex, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 76, (for 1985), 481-513. KNIGHT,I. & BOYCE,W. D. 1987. Lower to Middle Cambrian terrigenous-carbonate rocks of Chimney Arm, Canada Bay: lithostratigraphy, preliminary biostratigraphy and regional significance. Current Research (i987), Newfoundland Department of Mines and Energy, Mineral Development Division, Report 8% l, 359365. KNOLL, A. H. & SWETT, K. 1987. Micropalaeontology across the PrecambrianCambrian boundary in Spitsbergen. Journal of Paleontology, 61, 898-926. LONGMAN,C. D., BLUCK,B. J. & VAN BREEMAN,O. 1979. Ordovician conglomerates and the evolution of the Midland Valley. Nature, 280, 578-581. MAGUIRE, P. K. H. 1987. CHARM II--A deep reflection profile within the central England microcraton. Journal of the Geological Society, London, 144, 661-670. MAX, M. D., BARBER,A. J. & MARTINEZ,J, 1990, Terrane assemblage of the Leinster Massif, SE Ireland, during the Lower Palaeozoic. Journal of the Geological Society, London, 147, 1035-1050. MUIR, M. D., BLISS,G. M., GRANT, P. R. & FISHER, M. J. 1979. Palaeontological evidence for the age of some supposedly Precambrian rocks in Anglesey, North Wales. Journal of the Geological Society, London, 136, 61-64. ODIN, G. S., GALE,N. H. & DOPE, F. 1985. Radiometric dating of Late Precambrian times. In: SNELLING, N. J. (ed.) The Chronology of the Geological Record. Geological Society, London, Memoir, 10, 65-72. PALMER,T. J., MCKERROW,W. S. & COWIE,J. W. 1980. Sedimentological evidence for a stratigraphical break in the Durness Group. Nature, 287, 720-722. PATCHETT,P. J., GALE,N. H., GOODWIN,R. & HUMM,M. J. 1980. Rb-Sr whole-rock isochron ages of late Precambrian to Cambrian igneous rocks from southern Britain. Journal of the Geological Society, London, 137, 649-656. PEAT, C. 1984. Comments on some of Britain's oldest microfossils. Journal of Micropalaeontology, 3, 65-71. PHARAOH, T. C., MERRIMAN, R. J., WEBB, P. C. & BECK1NSALE,R. D. 1987. The concealed Caledonides of eastern England: preliminary results of a multidisciplinary study. Proceedings of the Yorkshire Geological Society, 46, 355-369. PHILLIPS,W. E. A., STILLMAN,C. J. & MURPHY,T. 1976. A Caledonian plate tectonic model. Journal of the Geological Society, London, 132, 579-610. POWELL,C. M. 1989. Structural controls on Palaeozoic basin evolution and inversion in southwest Wales. Journal of the Geological Society, London, 146, 439-446. ROWELL, R. J., ROBISON,R. A. & STRICKLAND,D. K. 1982. Aspects of Cambrian agnostid phylogeny and chronocorrelation. Journal of Paleontology, 56, 161182. -
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RUSHTON,A. W. A. 1974. The Cambrian of Wales and England. In: HOLLAND,C. H. (ed.) Cambrian of the British Isles, Norden and Spitsbergen (Lower Palaeozoic rocks of the World, 2), 44-121. Wiley, London. 1978. Fossils from the Middle-Upper Cambrian transition in the Nuneaton district. Palaeontology, 21,245-283. -1983. Trilobites from the Upper Cambrian Olenus Zone in central England. Special Papers in Palaeontology, 30, 107-139. - - , HAMBLIN,R. J. O. & STRONG,G. E. 1988. The Croft Boreholr in the Lilleshall Inlier of north Shropshire. Report of the British Geological Survey, 19, no. 3. - - & MOLYNEUX,S. G. 1990. The Withycombe Formation (Oxfordshire suberop is of early Cambrian age). Geological Magazine, 127, 363. & PHILLIPS,W. E. A. 1973. A Protospongia from the Dalradian of Clare Island, Co. Mayo, Ireland. Palaeontology, 16, 231-237. - - & TRIPP, R. P. 1979. A fossiliferous lower Canadian (Tremadoc) boulder from the Benan Conglomerate of the Girvan district. Scottish Journal of Geology, 15, 321-327. SHERGOLD,J. H. & BRASIER,M. D. 1986. Proterozoic and Cambrian phosphorites-specialist studies: biochronology of Proterozoic and Cambrian phosphorites. In COOK,P. J. & SHERGOLD,J. H. (eds) Phosphate deposits of the World. Volume 1. Proterozoic and Cambrian phosphorites. Cambridge University Press, 295-326. SMITH, D. G. 1977. Lower Cambrian palynomorphs from Howth, Co. Dublin. Geological Journal, 12, 159-168. -1981. Progress in Irish Lower Palaeozoic palynology. Review of Palaeobotany and Palynology, 34, 137-148. SWETT,K., KLEIN,G. D. & SMIT,D. 1971. A Cambrian tidal sand body--the Eriboll sandstone of Northwest Scotland: an ancient-Recent Analog. Journal of Geo, Iogy, 79, 400-415. & SUIT, D. E. 1972. Cambro-Ordovician shelf sedimentation of western Newfoundland, northwest Scotland and central East Greenland. 24th International Geological Congress, Montreal, 1972, Section 6, 33-41. TAYLOR, K. & RUSHTON, A. W. A. 1972. The pre-Westphalian geology of the Warwickshire Coalfield. Bulletin of the Geological Survey of Great Britain, (dated 1971). THEOKRITOFF,G. 1979. Early Cambrian provincialism and biogeographic boundaries in the North Atlantic region. Lethaia, 12, 281-295. 1981. Early Cambrian faunas of eastern New York State--taphonomy and ecology. Short Papersfor the Second International Symposium on the Cambrian System, 1981. United States Department of the Interior, Geological Survey Open File Report 81-743, 228-230. THOMAS, A. T., OWENS, R. M. & RUSHTON, A. W. A. 1984. Trilobites in British Stratigraphy. Geological Society, London, Special Report, 16. THORPE, R. S., BECKINSALE,R. D., PATCHETT,P. J., PIPER,J. D. A., DAVIES,G. R. & EVANS,J. A. 1984. Crustal growth and late Preeambrian-early Palaeozoic plate tectonic evolution of England and Wales. Journal of the Geological Society, London, 141, 521-536. TIETZSCH-TYLER,D. & PHILLIPS,E. 1989. Correlation of the Monian Supergroup in NW Anglesey with the Cahore Group in SE Ireland (extended Abstract), Journal of the Geological Society, London, 146, 417-418. TUCKER, R. D. & PHARAOH,T. C. 1991. U-Pb zircon ages for late Precambrian igneous rocks in southern Britain. Journal of the Geological Society, London, 148, 435--444. VANGUESTAINE,M. 1974. Esprces zonales d'acritarches du Cambro-Trrmadocien de Belgique et de l'Ardenne Franeaise. Review of Palaeobotany and Palynology, lg, 63-82. 1978. Critrres palynostratigraphiques conduisant ;i la reconnaissance d'un pli couch6 Revinien dans le sondage de Grand-Halleux. Annales de la Socidtd Gdologique de Beige, ltlO, 249-276. WHITCOMBE,D. N. & McGUIRE, P. K. H. 1981a. A seismic refraction investigationof the Charnian basement with granitic intrusions flanking Charnwood Forest. Journal of the Geological Society, London, 138, 643--651. & -1981b. Seismic refraction evidence for a basement ridge between the Derbyshire Dome and W of Charnwood Forest. Journal of the Geological Society, London, 138, 653-659. WILLS, L. J. 1978. A palaeogeological map of the Lower Palaeozoicfloor below the cover of upper Devonian, Carboniferous and later Formations. Geological Society, London, Memoir, 8. -
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3 5 ,
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Silurian M . G. B A S S E T T ,
B. J . B L U C K ,
R. C A V E ,
The history of establishment of the Silurian System (and hence of the Silurian Period) and its principal divisions has been reviewed by Cocks et al. (1971) and Holland (1984, 1989a). The System itself, three of the four Series into which it is divided, and the Stages within these three, are all based upon localities in Britain. The names of all the divisions shown in Fig. 1 have been ratified by the International Union of Geological Sciences. The two major divisions, Lower Silurian and Upper Silurian, remain informal, but they are useful when degree of precision in correlation does not allow reference at the level of Series (Holland 1989a). The boundary stratotypes are all described in Holland & Bassett (1989). Graptolite Biozones have proved especially useful for international correlation in the Silurian and the sequence of Biozones used in Britain is shown in Fig. 1. Correlation of the Biozones with the global Standard Series and Stages is shown approximately, but is not known with exactitude in all cases. Standard
Series
Standard Stage
graptolite biozones
Ludfordian
bohemlct~ Zelnt~xzrdlnertsls tumeaeens
P~DOLI o . 84 O.
scanlcn~s
ni~asonl Zudensis
z
nassa
Zundorent
<
e~Zesae
Sheinwoodian
or Z3 ...J 09
o -J
Zinnarssont
rlgldus
,_
Telychlan
Aeronian
Rhuddanian
r[eeurtonena[s .mrehtsont centri~ugl~ erenu~ata grlestonlensls erispus turrlcu~atus sedg~teklt convo~utus ~eptotheca magru~ trfangu~atus eyphus aclnaces
atavus acumtnagus
Fig. 1. Silurian Global Standard Stratigraphy and graptolite biozones. Geodetically the position of the British Isles in the Silurian remained astride the 25~ latitude, a position it had reached in the later Ordovician (Briden et al. 1973); movement was confined to rotation. During this time the south magnetic pole wandered, in terms of present geography, from near to southwest Africa, to the Horn of Africa in early Silurian times, then back along the equator to west Africa/Brazil at the end of the Silurian. CHH, RC
LLANDOVERY The Llandovery Epoch was characterized by marine transgressions. Thus in the early Llandovery the English Midland Platform, much expanded by the late Ordovician regression, was flanked to east and west by narrow shelfseas, whereas by the end of the epoch it was almost completely drowned by a significant shallow sea. On the opposite side of the remnant Iapetus Ocean, marine transgressions northward from the Mayo Midland Valley Basin encroached upon Grampian terrane. The transgressions were partly, maybe largely, eustatic and the agencies effecting them climatic and tectonic, with both operating together in the early part of the epoch. Volcanism was minor except along the northern edge of Pretannia from the Mendips to southern Ireland (Cope & Bassett 1987) and at the source of the bentonites of the Southern Uplands Birkhill Shale. The climatic effects towards the end of the Ordovician were those attendant upon the growth of a polar ice cap over what is now the Sahara. Sea level fell perhaps by 100 m (Brenchley 1984) and rose again when the icecap melted towards the end of the Ordovician. The resulting marine transgression was a worldwide event and it continued well into the Llandovery (~3'phus times); the later stages were probably due to a tectono-eustatic rise of sea level, a process which probably sustained the later Llandovery transgressions, too. It is possible that these repeated tectono-eustatic rises stemmed from closure or partial closure of the Iapetus Ocean, or from a ridge-trench collision (Campbell 1984) which ended southeastward subduction. 9 1992 The Geological Society
C. H . H O L L A N D
& J . D. L A W S O N
The end of the Ashgill coincided closely with the collapse of the late Ashgill ice-cap, and the basinal rocks of the persculptus Biozone reflect a sharp change in the depositional environment. The southern British Isles basins had a form which continued to the end of the Silurian, and thus the Ashgill Epoch is revealed more as the antecedent of the Llandovery than a descendant of the Caradoc. Localized tectonic activity occurred widely during, the Llandovery; in Pretannia to the south, along the eastern margin of the Welsh Basin, in the closure of Iapetus leaving a remnant linear basin, and in the rising of the southern margin (Cockburnland) of the Midland Valley Basin (Bluck 1983). RC
Sla: Southern British Isles: latest Ashgill-early Llandovery The time represented by the persculptus and acuminatus biozones signalled the start of the Llandovery marine transgression, and for this reason was chosen for the Atlas. As the base of the Silurian is formally defined at the base of the acuminatus Biozone (Cocks et al. 1984), part of the evidence used graptolite zones with the shelly sequences is still not accurately accomplished, the timing of events is not precise and is open to amendment. For example, it is impossible to correlate the shelly shelf sequences of Garth and Llandovery on the southeast flank of the Towy (Twyi) Anticline with the graptolitic basinal sequence some 6 km away on the northwest flank. The persculptus mudstones thus cannot be mapped into the shelly sequence of sandstones and mudstones to afford any sort of physical link. Even where the Towy Anticline plunges steeply northward at Abbeycwmhir (Roberts 1929) the persculptus Biozone strata (together with higher beds) disappear northward from the sequence on the western flank of the anticline before sandstones containing a Hirnantia fauna appear around the closure. At this time the deeper water depositional basins of Wales, the Lake District and Tornquist Sea persisted on either flank of the Midland Platform of England. The shallow shelf-seas were narrow, but were widening with the transgression, so that terrestrial detritus carried from the Platform in rapid run-off rivers were trapped in the marine shelf environment behind the advancing shoreline. The basins and their slopes, suddenly starved of coarse detritus and concomitant high energy turbidity currents, accumulated fine mud from low density turbidity currents and a rain of carbonaceous pelagic debris. Occasional floods of coarse detritus invaded the generally anoxic basins through restricted basin margin submarine channels. Small base-of-slope bodies of gravel and sand formed, e.g. along the southeastern margin of the Welsh Basin (Drew & Slater 1910; Kelling & Woollands 1969, pp. 262-264 and possibly Lockley 1983). In this sector the margin was sharply defined along or just east of the site of the Towy Anticline. North of Abbeycwmhir the position of the basin margin is less well constrained, for the early Llandovery is not exposed in crucial areas; a line from there passing east of Bala and Llandudno is an approximate position. East of this line shelf-sea sediments, often immature, poorly sorted sands, muds and gravels, were deposited somewhat patchily. The faunas are dominated by brachiopods but do not provide precise indications of age; it seems that on the east side of the Berwyns the marine invasion of the shelf reached Meifod early in the Rhuddanian, but did not reach the Welshpool to Berriew line (the Severn Valley) until late in the Rhuddanian. So, while there are respects in which the Severn Valley here is the structural extrapolation of the Towy Anticline, it did not function palaeogeographically as it did near Garth, Llandovery and probably Haverfordwest where it separates shelly, shallow marine shelf facies from graptolitic basinal facies. Indeed in acuminatus Biozone times there was land at Welshpool and Berriew, while at Garth, Llandovery and Haverfordwest there was a deepening marine condition, a condition which had been continuous from at least mid Ashgill times. Rapid sedimentation of transverse downwarps in the Platform edge was under way in places, e.g. Llandovery and Garth, so the actual acuminatus Zone shoreline was located south of Haverfordwest, east of Llandovery, probably just west of Builth and through Welshpool. Two palaeogeographical enigmas relate to the northern part of this area. One is the problem of the divide between graptolitic and shelly facies at the north end of the Towy Anticline and in the west Berwyns (Fig. 2). Along this line the pre-griestoniensis Llandovery sequence is absent or largely absent. In part of the west Berwyns the griestoniensis Biozone rests upon the persculptus Biozone and in the Abbeycwmhir area of the Towy Anticline the former zone rests upon Ashgill strata, Hirnantian and probably older. The sediments immediately above the break were graptolitic muds,.not the deposits of a transgressive shoreline. The break is thus a non-sequence by non-deposition or by sliding and slumping within a submarine environment. 37
SILURIAN
shelf. To the north, turbidite deposition was continuous from the Ordovtctan into the Wenlock. Further north, at Balbriggan (Rickards et al. 1973) and marginal to the lapetus Suture at Tomgraney, Co. Clare (Rickards & Archer 1969), basin-plain graptolitic muds were deposited at least as early as the acuminatus Biozone. The Tomgraney exposure may well lie north of the lapetus Suture and the deposits therefore may have been oceanic pelagic muds now associated with the margin of Laurentia. In northwest England anoxic graptolitic muds were accumulating in the western Lake District comparable with, and probably in continuity with, those of the Welsh Basin and southern Ireland9 In the eastern Lake District Cross Fell and the Howgill Fells (Rickards 1978; Burgess & Holliday 1979, and at Horton in Ribblesdale, Arthurton et al. 1988) black mudstones have at their base very calcareous mudstone, shelly in places, or a hard limestone representing probably the persculptus and acuminatus biozones. These deposits are thin, of shallow marine origin; the hard limestone with little clastic detritus (e.g. in the Howgill Fells) may have been a hardground which formed on the northern peninsular area of the Midland Platform at the end of the glacioeustatic marine low, The condensed sequence rests in places on sandy Hirnantian deposits and in the Cross Fell Inlier (Burgess & Holliday 1979) it has yielded Hirnantia sagittifera (Ingham & Wright 1972); this is probably compatible with the supposed persculptus age and reveals that there was little, if any, prior non-deposition in places. At Austwick, however (Arthurton et al. 1988), a non-sequence does separate Hirnantian strata from late Aeronian or Telychian rocks, so that attenuation or non-deposition did occur in places. The late Hirnantian/early Rhuddanian saw the growth of a major N-S fault, the Brathay Fault (Rickards 1978) on the east side of the Lake District and the limestone facies was developed only to the east of it. Furthermore the greatest attenuation of the early Llandovery sequence occurs close to the fault along its footwall. This may be a horst (Rickards 1978) but footwall uplift on an extensional down-to-basin fault (cf. Jackson & McKenzie 1983) is another possibility. If the eastern fault of the horst be discounted then the submarine slope down towards the Howgill Fells 'basin' had to be steep enough to promote slumping (R. B. Rickards pers. comm.). The Brathay Fault, however, may have been only one of a set of approximately N-S Llandovery extensional faults; for instance large gaps in the Llandovery sequence commonly lie over or just east of approximately N-S structures such as the Brathay Fault, the Western Berwyns Lineament, the Severn Valley, the Church Stretton Fault (Presteigne) and the north end of the Towy Anticline. Whereas the Taconic folding of the Shelve-Builth Basin followed the cessation of southward directed subduction, these extensional elements may be related to collision and cessation of northwestward directed subduction during the Llandovery (Murphy & Hutton 1986). RC
Fig. 2. Structural elements influential in Llandovery deposition in Wales and Welsh borderland (partly after Temple 1988). Furthermore the Llandovery sequence is more complete not only to the west, in the Welsh Basin, butalso to the east. In the east Berwyns the early Llandovery sediments are shelly; NW of Builth (e.g. Maesgwynne and Trecoed; Baker & Hughes 1979) shelly muds and sandstones are considered to extend down to the erispus Biozone; near Llanddewi Ystradenni pale coloured shelly and graptolitic mudstones are reportedly as low 'Middle Llandovery' (N. Kirk, MS). This non-sequence probably represents parts of the early and mid-Llandovery submarine upper slope and distal shelf-edge (Fig. 3). The second enigma is the depth index of the early Llandovery faunas east of the Berwyns around Welshpool. A progressive marine invasion of subdued terrain suggests shallow water, but the Meifodia-Clorinda-Strieklandia fauna of the Powis Castle Conglomerate is indicative of relatively deep water. The apparent contradiction points to a steep shore-face, controlled possibly by extensional faulting along the line of the Severn Valley (Fig. 3). From south Wales the shoreline extended across St George's Channel skirting the south coast of Eire. Llandovery turbidites, mud-sand rhythmites, commonly with base-missing Bouma sequences, occur in southeast Ireland (Briick et al. 1979); and in the extreme south base-of-slope conglomerates (Ballyhest Member) occur, comparable with those on the southeast margin of the Welsh Basin. A precise age for these is not known (they may be mid Llandovery rather than early) but they coarsen and thicken southward suggestive of a steep basin-margin slope and a narrow marine
NW BALA
Eastern
limit of earlyLMndoverygraptoliticmud
Slb, S2a: Southern British Isles: Llandovery (Aeronian) For most of the Aeronian the palaeogeography changed little. Turbidites dominated in parts of the basins, e.g. the Derwenlas Formation in Wales, and 'distal mud' in the Stockdale Shales/Skelgill beds in the Lake District and Howgill Fells. The second transgressive pulse is attributable probably to a tectonic event and it commenced in convolutus times (Aeronian) but characterized more particularly sedgwickii times. Marine conditions spread rapidly across the subdued Midland Platform; its shoreline ran from south of the Malvern Hills up their west side to fringe the Precambrian rocks of Shropshire. There the Longmynd formed a southwestward extended peninsula with sub-littoral sands on its eastern side and an exposed shoreline of cliffs on its west (Bridges 1975). North of Shropshire the position of the shoreline is ill-defined. Geophysical evidence (Bott 1967) suggests that a salient of the Midland Platform projected into northern England, but the shoreline must have lain east or southeast of Ribblesdale and thence through eastern England west of boreholes near the east coast which penetrated Llandovery turbidites,
Western limit of early
W E S T E R NBERWYN HILLS
SE
Llandoverymudsand& gravel MEIFOD
I
WELSHPOOL
I
I
. ..--".... B. . . .
9 ""
.''~-~Ga~l~ .'"~irnant Lst.,
/
mud~
f
,fi~tonienais
L A T E
'c...... andst0he'.~aio .........
L L A N D O V E R Y
w ~ ~ , ~ /'
I I I I I~aieAlshJIIre r!se,4n ! I I
ASHGILL-
T
T I ~ O ' ~ K i l o m e t r e
s
1p~~
;' T
Fig. 3. Non-speculative model of stratigraphical events in the southern Berwyn Hills. Vertical ruling represents non-sequences.
39
40
SILURIAN
West of the Midland Platform the shoreline bounded the north side of Pretannia (Cope & Bassett 1987) through South Wales to south Pembrokeshire, having migrated a little southwards beyond its early Llandovery position. This suggests that the margin of Pretannia was steep at the time or was tectonically positive. There was a very active volcanic centre in St George's Channel, off Skomer Island (Ziegler et al. 1969) extruding basalt and rhyolitic domes eastward and southward (Bridges 1976; Cope & Bassett 1987; Walmsley & Bassett 1976). These also shed detritus southwards creating off-shore barrier islands and shallow marine lagoons. Close by to the north, must have lain the basin, yet the products of volcanism are not obvious in the contemporary deposits there. Immediately afterwards the Pretannia shoreline regressed northwards, for deposits of the turriculatus Biozone are absent in Pembrokeshire. This event coincides with the shedding of the first of the northward directed lobes of coarse, commonly feldspathic and often high-matrix turbidite sands into the mid-Wales basin, i.e. the Aberystwyth Grits (Cave & Hains 1986). It was at this time, therefore, that Pretannia took over from the eastern shelf as main supplier of coarse detritus to the basin probably in direct consequence of tectonic and volcanic activity along its northern edge. During the Aeronian the Caban Canyon of Kelling & Woollands (1969) had continued to entrain coarse extra-basinal high-matrix gravels basinward from the east, past Rhayader, but the sedgwickii-turriculatus transgression isolated the canyon-head from its supply of detritus, and shut it down. Slumped mudstones overlain by coarse and pebbly sandstones (J. A. Zalasiewicz, MS) indicate a brief resumption and eastward shift of canyon activity on this eastern margin in iurriculatus times, but the main input to the basin from the east was of high frequency, low density, mud-dominated turbidites produced possibly by linear spill-over of shallow marine storm surges from the shelf, e.g. Devil's Bridge and Cwmsymlog formations and Rhayader Pale Shales, of which there are green and red facies. During this time the redox surface lay within these deposits, below the benthic boundary layer, and the basin floor was colonized by a burrowing and grazing benthos leaving traces such as Nereites and Dictyodora. In turriculatus times northeastern Wales saw the deposition of non-shelly oxic muds punctuated by brief anoxic algal/graptolitic blooms, while the shelly mud/sands facies advanced eastward beyond the earlier shoreline. Basinal muds, often graptolitic and anoxic, also characterized the western Lake District (Rickards 1978) and central Ireland (Balbriggan; Rickards et al. 1973). Presumably the marine transgression seen in Wales and the Borderland occurred here too, but minor influxes of sand recorded in the western Lake District and at Balbriggan suggest the appearance of a more active or proximal source area. The Brathay Fault was not active at this time and muds with graptolites were deposited across it at least as far as Ribblesdale but these eastern sequences are thin, presumably platform mud with a shelly calcareous component in the early Telychian. RC
S2b: Southern British Isles: (Telyehian) The last of the Llandovery marine transgressions occurred in griestoniensis time. In the English Midlands and the Welsh Borderland the siliciclastic, mainly mud/sand deposition which had been established everywhere west of the Malvern Line then extended over the Midland Platform (Ziegler et al. 1968). The shelf-sea sediments, derived presumably from Pretannia to the south, were distributed by storm surges. Water was mostly shallow, though in places below wave-base (Benton & Gray 1981). Near Tortworth, in the northern Pretannia orogenic zone, two basalts were extruded, both probably in griestoniensis time. The earlier one flowed directly over Tremadoc rocks and supports red mudstones on and near its surface. Here, therefore, may have been an early griestoniensis land surface poised for inundation. Towards the western edge of the Midland Platform, along the line of the Church Stretton Fault at Old Radnor and Presteigne, no deposits of griestoniensis age are preserved and coarse quartzose sands were deposited on Longmyndian rocks in crenulata time. Apparently this tract remained positive, possibly land, up to crenulata time thus maintaining a barrier which trapped detritus on the shelf to the east. To the west, the Welsh Basin remained little changed. It received further major influxes of coarse detritus from Pretannia to the south, producing vertically stacked lobes of turbiditic sand within a SSW-NNE depositional system some 100 km long (Smith 1987). Conditions were often anoxic, so that interturbiditic, hemipelagic muds were commonly graptolitic and carbonaceous. Dysaerobic and oxic intervals also occurred, developed as bioturbated mudstones, especially in the thinner bedded sequences on the east side of the basin and lateral to the coarse sand lobes. The eastern margin of the basin continued to suffer tectonically affecting bathymetric gradients (particularly at the north end of the Towy Anticline) and disturbing sediments and sedimentation in griestoniensis and crenulata times and even into the Wenlock. On the eastern flank of the anticline, east of Abbeycwmhir, mud was deposited directly upon an Ordovician substrate. In places (e.g. Henfryn) there was a rudaceous base, largely of a mass mudflow nature, containing late-Ordovician debris. On the western flank of the anticline the underlying deposits up to the crispus Biozone show slumping, but away from the anticline on both sides griestoniensis sediments were deposited upon earlier Llandovery deposits: It would seem, therefore, the griestoniensis mass-flows can have been nourished only from contemporary sediments high on the eastern flank of the anticline and from the Caradoc/
Ashgill substrate there. Since there appears to be no local shore or shallow facies in the area, except the bed of coarse cross-bedded sandstone within the Henfryn Conglomerate, these activities and the non-sequence must have been submarine. Over the western Berwyn Hills (Figs 2 & 3), mud deposition resumed during griestoniensis time in a similar rhythmic, dysaerobic and anoxic style; to the east however, where mud deposition had been much more continuous, the conditions were mainly oxic and the griestoniensis (and earlier) mudstones are red and green as well as grey. In crenulata time such conditions spread widely over Laurentia and northern Gondwana while along northern Pretannia the shoreface transgressed southwards once more, drowning the griestoniensis terrain of earlier Llandovery sediments and voicanics. In parts of east Wales the pale coloured late Llandovery muds supported a shelly benthos, e.g. near Welshpool, just west of the Severn--Towy Anticline lineament, and near Llandrindod Wells, just east of it. Although the age of these faunas may differ from place to place, the lateral passage from shelly to barren mudstones is abrupt. These circumstances along the shelf margin suggest that local shoaling, probably produced tectonically, reduced marine depths to within a sharply defined base of viability for the epifaunas. In northern England during griestoniensis time grey marine muds were being deposited. The Brathay Fault showed little sign of activity and banded muds, rather like those of the western Berwyn Hills, extended from the western Lake District across to the Howgill Fells, Cross Fell and Ribblesdale where distal turbiditic mud rhythmites were accumulating. Deposits in central Ireland (Balbriggan; Rickards et al. 1973) were very similar to those of the Lake District and it is likely that one large marine 'basin' extended across southern Ireland into the Lake District and Wales. The imprecisely dated turbidites of southeast Ireland (Briick 1972; Penney 1980) continued to form a fan, proximal in the southeast, extending distally over the basin-plain to the northwest. The westward directed currents indicated west of the Leinster Granite could have come from the Irish Sea land-mass, but equally they might have been bathymetric deflectants of a general northward flow, for no good evidence of the land-mass comes from the Welsh Basin at this time. RC WENLOCK The marine transgression eastwards and southwards in the late Llandovery led to a palaeogeographical situation in the southern part of the British Isles which comprised a basin to the west and an extensive shelf to the east, across the Midland Platform. This pattern prevailed through Wenlock times. The basin was, however, partially divided by an area of shallower water and perhaps an island, traditionally referred to as the Irish Sea Landmass. At the beginning of the Wenlock (not shown on the maps), localized deposition of carbonates in the Welsh Borderland led to the formation of the Woolhope Limestone. The more widespread and prominent development of carbonates across the Midland Platform is seen in the Much Wenlock Limestone Formation of the late Wenlock. Farther to the east, and with reference to the whole of the Wenlock, data from boreholes indicate the presence of deeper water and a connection with the open sea. There is no evidence for a Iapetus Suture in the Wenlock rocks of the British Isles, though information may have been lost along a subsequent fault following approximately the same line, and possibly with significant sinistral displacement. Definitive and general works on the Wenlock rocks of the British Isles, which also lead to other references, are as follows: Bassett et al. (1975) and Bassett (1989) (for the type and standard Wenlock), Bassett (1974) and Holland (1989b) (for England and Wales); Rickards (1978) (for Northern England); Holland (1981) (for Ireland); and Walton (1983) (for Scotland). CHH
S3a: Southern British Isles: Wenlock (Sheinwoodian) The early Wenlock successions of the central Irish inliers are commonly of poorly fossiliferous graptolitic shales and tubidites. Spores are present in the Slieve Bloom Inlier and that of Slievenamon. Richly graptolitic mudstones are seen at Balbriggan. Directional data are limited. In northern England, data from the Austwick Formation of graded greyish sandstone turbidites, alternating with mudstones, suggest a northwesterly flow and proximity to an east-southeasterly shoreline. Farther to the north-west in the Howgill Fells, the Lake District, and the Cross Fell inlier, is the distinctive Brathay Flags lithology of blue-grey laminated mudstones with calcareous nodules and bentonites. The Brathay Flags are richly graptolitic and yield orientated orthoconic nautiloids. Rickards (1978) has suggested that laminations running through the calcareous nodules indicate a reduction in thickness from the original mud of over 40 per cent. Since the earlier sedimentological work by Cummins (1957), using flute moulds well seen in the Central and North Welsh turbidites of the Denbigh Grits Group as directional indicators, it is necessary to recognize two troughs, the Denbigh Trough and the Montgomery Trough, which come together in the north. They were separated by the Derwen Ridge. There is some evidence of slumping on the western margin of the Montgomery trough. A thinner, more graptolitic sequence is seen near the North Welsh coast. A graptolitic facies of flaggy silty mudstones and shales is also present at the other side of the basin in the Builth district, where Elles did the original work on the graptolitic biostratigraphy of the Wenlock. On the shelf to the east are the muddy, more or less calcareous rocks
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
42
SILURIAN
generally referred to collectively as the Wenlock Shales. They are sparingly graptolitic but yield rich shelly faunas of brachiopods, trilobites, corals, etc. Thin bentonites are commonly present, as, for example, in the Wenlock Shales of the Ludlow anticline. The andesites and tufts of the Mendips were regarded by Ziegler et al. (1968) as of Wenlock age. The southern shoreline is shown as running north of Freshwater West and Freshwater East in Pembrokeshire (Dyfed), as the basal part of the Wenlock is missing here (Bassett 1974). What amounts to a very substantial knowlege of the pre-Permian floor was charted by Wills (1978). A number of boreholes for coal or water have penetrated Silurian rocks. The extent to which this southeastern marine area merged with the Welsh Basin and shelf remains uncertain. Exposures and boreholes in Belgium suggest a marine connection in this direction, CHH
S3b: Southern British Isles: Wenlock (Homerian) Through the late Wenlock, the palaeogeographical framework remains similar, but there are some additional sources of evidence and there are some important changes in the facies pattern. Conspicuous among the central Irish inliers is the relatively large Slieve Phelim-Devilsbit Mountain district (Doran 1974). Graptolites indicative of the Monograptus ludensis Biozone are actually confined to a small area in the northwestern part of the inlier, but the repetitive sequence of greywackes, laminated siltstones, and mudstones in the remainder of the inlier are confined to the immediately preceding Cyrtograptus lundgreni Biozone. A quiet water r6gime was interrupted by westerly flowing distal turbidity currents. Apart from graptolites and orthoconic nautiloids, other fossils are rare. Cope (1959) analysed those present to show that the greywackes carry derived thick-shelled brachiopods and crinoids, certain micaceous siltstones show a terrestrially derived association of plant fragments and a phyllocarid, whereas possibly benthonic brachiopods, small bivalves, and crinoids occur in the more common laminated siltstones. In the very small inlier of Knochshigowna Hill, to the north of the Slieve Phelim-Devilsbit inlier and to the west of Slieve Bloom, a similar facies of early Wenlock age is followed by conglomerates with sandy pockets containg a Homerian shelly fauna in which brachiopods are the dominant constituent (Prendergast 1972). It appears to have been derived from a shallow water environment, though the state of preservation of the fauna does not indicate prolonged transport. The narrow inlier of the Cratloe Hills to the north of Limerick City also yields a pebbly sandstone with Homerian shelly fossils, interrupting the usual sequence of sparsely graptolitic laminated siltstones and sandstones (Holland et al. 1988). There is thus some evidence for a northerly directed palaeoslope. Confirmatory evidence for the palaeogeography is seen in the very rich Homerian shelly faunas of the Dingle Peninsula (Holland 1988). There are associated volcanics, chiefly acid in composition, including very well displayed ignimbrites and pyroclastics. Finally, to the northeast at Balbriggan the largely unfossiliferous greywackes of the Skerries Formation contain fragmentary graptolites of very late Wenlock and possibly younger age. There is evidence of the former presence of Wenlock rocks in the occurrence of derived corals in the Carboniferous (or possibly Devonian) Peel Sandstone of the Isle of Man (Crowley 1985). The situation in northern England remains similar to that in the early Wenlock. The Middle Coldwell Beds are of Monograptus ludensis Biozone age. Their pale grey mudstones are unlaminated or thoroughly bioturbated. There are limestone lenses rich in trilobites and brachiopods, and also graptolitic bands of Brathay Flags lithology. In North Wales, the latest part of the Wenlock is represented by the Lower Nantglyn Flags Group (Warren et al. 1984), the background sedimentation of which is of 'ribbon banded mudstones'. In this facies mudstones alternate with laminated muddy siltstones yielding graptolites, orthocones, and bivalves. There are some lenticular beds of calcareous siltstone. The whole represents relatively quiet sedimentation, with the mudstones having arrived from weak turbidity currents and the calcareous siltstones as distal turbidites. Interrupting the ribbon banded mudstone sequence are the disturbed (slumped) beds of the Brynsylldy Formation and, at a generally lower level, the intensely bioturbated Upper Mottled Mudstone with its shelly fauna. Cummins (1959) gave detailed description of the Nantglyn Flags from the wider area from Central to North Wales. Their directional system of derivation was as for the earlier and coarser Denbigh Grits. At Marloes Bay in Pembrokeshire, the shallow water Grey Sandstone 'Series' yields Homerian fossils in its lower part. A little farther south at Freshwater West and Freshwater East the youngest Wenlock beds are already in Old Red Sandstone facies. At Cardiff, poorly fossiliferous sandstones and siltstones are succeeded by a thin red crinoidal limestone which appears to be equivalent to part of the Much Wenlock Limestone. According to Bassett (1974), the southern margin of the Wenlock Limestone Platform lay close to Usk and Tortworth, where arenaceous horizons appear. The Ware and Cliffe boreholes to the east show faunas and lithologies of Wenlock Shales type. In the richly fossiliferous Much Wenlock Limestone the bioherms are best developed at the seaward fringe of the reef belt, and there is also an upward increase in reefal material as the clay particles decrease. The frame builders were especially tabulate corals, together with stromatoporoids, branching rugose corals, and fenestellid bryozoans. Stromatolites, laminar tabulate corals, stromatoporoids, algae, and bryozoans acted as reef binders. There
were associated crinoids, brachiopods, ostracodes~ and gastropods. The bioherms have an average diameter of 12 m, with maximum thickness of some 45 m. There are various inter-reef and off-reef facies of argillaceous and crinoidal limestones. The Wenlock Limestone at Dudley appears to have been deposited earlier than elsewhere, i.e. at the time of the preceding Cvrtograptus tundgreni Biozone, Again a connection to open sea is shown in the east, Siveter & Turner (1982), described a microvertebrate assemblage from the Wenlock rocks of the Tortworth inlier, which has Baltic and Siberian affinities. Another hint of such wider connection from west to east is the occurrence of the Bohemian tabulate coral Stelliporella in the Homerian rocks of the Dingle Peninsula. CHH LUDLOW After an initial rise in sea level the dominant theme in Ludlow times was one of marine regression, leading to the restricted marine and fluviatile conditions of the Pfidoli Epoch. The Welsh and Lake District basin became silted up and shallow. Terrestrial red beds were developed well before the end of the epoch in Pembrokeshire and southwest Ireland. In Scotland there is some palaeontological evidence for Ludlow rocks (Selden & White 1983). Radiometric dates on lavas of the Lower Old Red Sandstone (Thirlwall 1988) support the long-held view that marine influenced alluvial deposition was taking place there in Ludlow times.
S4a: Southern British Isles: Ludlow (mid Gorstian) Over the Welsh Borderland there was a marked transgression in early Gorstian times indicated by mudstones with a diverse fauna of dominantly small brachiopods. The basin areas accumulated graptolitic muds at this time. The greatest depth appears to have been in mid Gorstian times when graptolitic muds were also deposited over the outer shelf region around Ludlow. Over most of the shelf, however, shelly muds (the Glassia obovata Association of Watkins 1979) were deposited. Bentonites are common but the source of the original volcanic ash is not known; a more distant source than the Mendips seems likely, perhaps in central Europe, Gorstian deposits are absent near Gorsley, south of the Woolhope Inlier, and are thin in the May Hill Inlier but thicken southwards in the Brookend Borehole indicating that the thinning is due to a local uplift and not an approach to a shoreline. There is little direct evidence of the source of the shelf sediments: being a time of maximum transgression, probably little of the Midland Platform was emergent. The dominant source of material is presumed to be Pretannia to the south (Cope & Bassett 1987). There is no borehole evidence of Gorstian rocks in eastern England but it is postulated that shallow seas covered this area. West of the Church Stretton lineament there is a fourfold increase in thickness and a marked change of facies in the Montgomery Trough of the Welsh Basin. The main stratigraphical unit is the Bailey Hill Formation which consists of alternations of hemipelagic carbonaceous laminae and calcareous siltstone units which Bailey (1969) interpreted as turbidites. Tyler & Woodcock (1987) have suggested an alternative origin as distal storm deposits from the shelf, deposited below wave-base in the deeper water of an outer shelf and not a basin. One difficulty in this interpretation is the rarity of storm deposits in the mid Gorstian rocks of the Welsh Borderland area: storm deposits are, however, very common in the succeeding Ludfordian rocks but do not result in pseudo-turbidites in the east-central Wales area. Tyler & Woodcock suggest that the main axis of the Welsh Basin lay farther to the west. Bailey, however, interpreted the northward flowing currents as turbidity currents flowing along the axis of a trough; this model is supported by the occurrence of slumped shelly mudstones around Builth which moved northwestwards from the shelf and slumped siltstones in the Clun Forest areas which moved southeastwards towards the presumed axis. Despite complications afforded by the Clun Forest area, the evidence seems to support the presence of a locally uplifted submarine area west of Clun Forest. It is indeed probable that the main axis of the Welsh Basin did lie well to the west; perhaps, as suggested for the Wenlock, there was a western trough and an eastern trough. In North Wales (Warren et al. 1984) the Upper Nantglyn Flags and much of the Elwy Group are of this age. The Nantglyn Flags comprise ribbonbanded mudstones with laminae of silt (possibly distal turbidites) alternating with the autochthonous hemipelagic muds. In the Elwy Group greywacke siltstones and sandstones of probable turbiditic origin provide evidence of transport from the west, along the axis of the Denbigh Trough. Penecontemporaneous slumps are common; the axes strike W-E and the evidence favours sliding from the north. As yet, no reinterpretation of the Denbighshire sediments as being storm generated has been attempted. The Irish Sea Horst is presumed to have persisted to the west from evidence before and after this epoch, forming the western boundary of the Welsh Basin, but the land area is shown as small because of the transgression. To the southwest of the Montgomery Trough, the Towy anticlinal area accumulated detritus brought northwards from a delta forming on the northern margin of Pretannia. Farther west the Ludlow rocks are cut out by an unconformity at the base of the Old Red Sandstone, but in Pembrokeshire the Gorstian is considered to be represented by the deltaic and intertidal deposits of the Gray Sandstone Group and the fluviatile facies of the lowest Old Red Sandstone.
43
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SILURIAN
The similarity of facies and faunas suggests that the Welsh Basin and the Lake District basin were connected. There, the laminated muddy siltstones of the Upper Coldwell Beds are succeeded by the thick Coniston Grits; these have been interpreted as turbidites with evidence of derivation mainly from the northwest. Furness (1965) suggested that the abundance of spilite and chert fragments in the Coniston Grits indicated a derivation from the Ballantrae Volcanic Group of southern Scotland. Alternatively a marginal arc of Ballantrae type on the northern edge of the Southern Britain plate is postulated here. In Ireland (Holland 1981), there are greywackes around Balbriggan and it is assumed that the graptolitic siltstones and shales of the Wenlock rocks of south-central Ireland persisted into the Gorstian. In the Dingle area of southwestern Ireland graptolitic shales pass westwards into calcareous siltstones with a shelly fauna, mainly brachiopods, similar to that of the Welsh Borderland shelf. JDL
South Wales lay close to the northern margin of Pretannia and suffered greater clastic input. The Usk Inlier displays the 'normal' nodular silty limestones on the east but there is a westward decrease in lime content. A little to the south at Cardiff (Waters & Lawrence 1987), poorly calcareous shelly siltstones dominate. Farther west, near Llandeilo, the postulated delta of mid Gorstian times had prograded northwards to give the conglomerates and quartzitic sandstones of the Trichrfig Beds (Potter & Price 1965), with red beds indicating periodic exposure. Marginal marine beds are characterized by a Lingula-bivalve-gastropod association suggesting salinity variations which discouraged articulate brachiopods and other stenohaline forms. Northwards along the Towy Anticline the sediments fine to siltstones, still poorly calcareous, and brachiopods become commoner. Outside the area of this map (i.e. Pembrokeshire, Lake District, Ireland) the conditions of depositon were similar to those of mid Gorstian times. JDL
S5a: Southern British Isles: (early Ludfordian) S4b: Southern British Isles: Ludlow (late Gorstian) In late Gorstian times a gradual change took place in the Welsh Borderland area from a muddy to a carbonate shelf: the change from 'Lower Ludlow Shales' to 'Aymestrey Limestone' in the older terminology. The Lower Bringewood Formation (Atkins 1979) comprises mainly bioturbated calcareous siltstones characterized by a fauna of large brachiopods, particularly strophomenids. The Mesopholidostrophia laevigata Association (Watkins 1979) is dominant but Atkins also recognizes a shelf-edge Shagamella ludloviensis Association and an inner shelf Atrypa reticularis-Sphaerirhynchia wilsoni Association. The map mostly covers the succeeding Upper Bringewood Formation, the most calcareous part of the shelf Ludlovian. At the shelf-edge along a N-S band from Craven Arms to Aymestrey bioclastic limestones are well developed, showing cross-bedding in places. Shell banks crowded with disarticulated valves of Kirkidium knightii (the K. knight# Association of Watkins & Aithie 1980) confirm the high-energy shallow water conditions. At Aymestrey, the limestone thins and passes rapidly into calcareous siltstones and then into laminated siltstones involved in westward slumping. The main shelf region seems to have been protected by the shelf-edge ridge, although there is no evidence of any of the long-standing positive blocks being emergent and undergoing erosion. In this quieter water the M. laevigata Association is dominant in nodular silty limestones. In the inner shelf area around the Malverns the influx of fine silts was increased and an Atrypa reticularis-coral Association occurs (Watkins & Aithie 1980). The near shore aspect is confirmed by the complete absence of the Upper Bringewood Formation (and other formations) in the southern Woolhope Inlier and the May Hill Inlier. Uplift in the Gorsley area (Lawson 1954) evidently resulted in non-deposition and/or erosion. South of the Bristol Channel, however, the nodular silty limestones reappear in the Brookend Borehole. The suggestion by Mohamad (1981) that much of the back-barrier shelf was intertidal seems untenable in view of the abundant articulate brachiopods, corals, crinoids and bryozoa. The shelf-edge was to the east of the line of the Church Stretton Fault for most of its traceable course. There are northwesterly directed slumps around Knill and New Radnor indicating persistence of a basinal area, not necessarily of great depth. Slumps in the Clun Forest area moved towards the southeast, again suggesting the margin of a trough along which silt was being transported northwards into an area of hemipelagic muds. The fauna of graptolites and orthocones with a poor benthic fauna suggests that the bottom waters were not well oxygenated, and lay well below wave-base although not necessarily at great depth. The alternation of silt and mud gave the laminated siltstone facies (Holland & Lawson 1963; the Striped Flags of Kirk 1951) that persists northwards through the Long Mountain into Denbighshire. For the Bringewood formations together the thickness increases from c. 80 m on the main shelf to ~<500 m in the basin.
In early Ludfordian times the carbonate shelf environment was replaced by clastic silt sedimentation. The basal sediments of the Lower Leintwardine Formation are still limestones, well developed in a higher energy carbonate belt near the shelf edge, e.g. Leintwardine and Aymestrey, where they are succeeded by shelly laminated shales. The limestones form the top part of the 'Aymestrey Limestone' of the traditional classification but it is notable that the Kirkidium knightii-tabulate coral Association has disappeared. The succeeding 'Mocktree Shales' contain a less diverse benthic assemblage dominated by Shagamella minor and Dayia navicula and show a transition to the basin facies. At Leintwardine submarine channels descended westwards into the basin and slumping to the northwest is developed from Aymestrey southwards. Over most of the shelf the Lower Leintwardine Formation averages 50 m in thickness. Basal nodular silty limestones are overlain by bioturbated flaggy calcareous siltstones with thin limestone bands. Watkins (1979) called this the coquinoid siltstone facies containing his Sphaerirhynchia wilsoni Association. The fossils occur as disturbed neighbourhood assemblages often in monotypic lenses due to environmental stress (Cherns 1988) on a storm-influenced subtidal shelf. The inner shelf sees a thinning of the deposits, particularly near the Gorsley axis, and the common occurrence of shelly conglomeratic limestones, some of which show evidence of hardground formation (Cherns 1980); these conglomerates are well seen at May Hill, South Woolhope and in the Brookend Borehole but also occur in central England, Wenlock Edge, Malvern and Usk. They evidently indicate interrupted deposition and periodic erosion, mainly submarine. The formation as a whole is characterized by a Sphaerirhynchia wilsoni-Isorthis orbicularis Association over the inner shelf. In the higher beds thin layers of phosphatized clasts and fish remains occur, again indicating phases of non-deposition with the enrichment of organic phosphates by winnowing of fine sediment in the shallow warm seas. A low-energy environment with a low sedimentation rate is postulated and there is little evidence of the direction of transport or the source of the sediment. From the Welsh Borderland shelf into the Welsh Basin there is an abrupt change both of facies and thickness (from 50 m to 200 m, then to 450 m; Holland & Lawson 1963, p. 280)just east of the Church Stretton Fault. The laminated siltstone facies is dominant, representing a low energy environment of poorly oxic muds receiving repeated influxes of silt. The Lingula lata-Saetograptus leintwardinensis Association is dominant: Cherns (1979) indicated that L. lata had the infaunal mode of life of Recent lingulids but in deeper and muddier seas. At the western edge of the outcrop in Clun Forest slumping to the southeast persisted, with a northerly movement of sediment along "~he presumed axial region of the trough. In South Wales, along the southeastern flank of the Towy Anticline, poorly calcareous flaggy siltstones and fine sandstones occur, presumably
Fig. 4. Facies relationships in the Ludlow Series of South Wales and the Welsh borderland (from Holland & Lawson 1963). Key: I, shelly olive mudstone facies; 2, muddy siltstone facies, 3, graptotitic shale facies; 4, laminated siltstone facies; 5, turbidite siltstone facies; 6, turnescenssiltstone facies, 7, shelly siltstone facies; 8, calcareous shelly siltstone facies; 9, slumped shelly siltstone facies; 10, strophomenid siltstone facies; 11, massive limestone facies; 12, flaggy limestone facies; 13, deltaic facies.
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SILURIAN
deposits of the delta front. The conditions were those of a moderate energy marginal sub-tidal environment. In Pembrokeshire, red sandstones are presumed to have been deposited partly in early Ludfordian times by rivers flowing northwards from the Pretannia massif. In North Wales the laminated siltstone facies persisted through the S. incipiens Biozone into the S. leintwardinensis Biozone, the higher parts of which are missing because of erosion. The same facies prevails in the Bannisdale Slates of the Lake District and a connection with the deeper water basin from Wales is assumed. Over most of Ireland (Holland 1981) there is no evidence but the pattern of earlier maps is followed by proposing a basin tongue stretching southwestwards across the centre of the country. In the Dingle Peninsula, however, shallow water calcareous siltstones are dominant with a benthic fauna in which the brachiopods suggest a connection with the Welsh Borderland shelf, although the precise route is not clear. The Upper Leintwardine Formation of the shelf is thin and though generally of similar facies to the lower formation it has a distinctive fauna including some trilobites and ostracodes which suggest a Baltic connection. This Shaleria-neobeyrichiacean Association facilitates correlation with Gotland and other areas. The basin facies has a widespread Aegiria grayiNeobeyrichia lauensis Association at the same level, and the neobeyrichiaceans also occur in the Lake District. JDL
S5b: Southern British Isles: (late Ludfordian) By late Ludfordian times an almost uniform shallow water facies had spread over the Welsh Borderland, east-central Wales and the Lake District and the area of deposition became more restricted with the continuing emergence of the surrounding land masses in the Midlands and Irish Sea areas. The Welsh basin was probably only slightly deeper than the shelf but remained a tectonically negative area with a great increase in thickness of Whitcliffe sediments from c. 40 m on the shelf to a maximum of about 400 m in the basin area of Clun Forest. The Upper Whitcliffe Formation of the main shelf comprises bioturbated olive-grey flaggy calcareous siltstones with shelly coquinoid lenses: wavy and ripple bedding are not uncommon with some thin cross-bedded units in the higher layers (Bradfield 1987). Alternations of bioturbated siltstones and shales with coarser laminated siltstones are interpeted as storm-influenced deposits. The shell beds may be the traction load of the initial coarse fraction deposit (Watkins 1979). Faunally the formation is characterized by a lowdiversity assemblage of brachiopods and bivalves: many of the early Ludfordian (Leintwardine) brachiopods have disappeared, perhaps unable to cope with an increased rate of sedimentation which would not have affected the infaunal bivalves. A Salopina-Protochonetes Association is dominant. The overall picture is of a proximal shelf subtidal environment with frequent storm sedimentation. Antia (1980) considered that the topmost beds were tidal. There was an increased sedimentation rate, increased energy level and increased proximity to source (Watkins 1979). Bailey & Rees (1973) postulate directions of derivation from the east but not from a nearby source. The facies is similar in the inner shelf but the Whitcliffe formations become very thin around the Gorsley uplift (2 m at Gorsley). To the south at May Hill, Tites Point and Brookend frequent interruptions in deposition are indicated by the occurrence of phosphatized pebble beds, layers of fish denticles ('bone-beds') and layers of euhedral biotite. There is also an easterly thinning towards the Midlands. Boreholes in eastern England (Stowlingtoft and Lowestoft) have yielded shallow water sediments of probable late Ludfordian age. To the west of the Church Stretton Fault system the equivalent strata increase rapidly in thickness due to more rapid deposition in a subsiding tectonic basin. The facies changes are, however, subtle: the siltstones are somewhat darker and muddier but still with wavy bedding, low angle crossbedding and lenses of shells. The brachiopods of the Salopina-Protochonetes Association are slightly but noticeably smaller than on the shelf and there is a more diverse molluscan fauna. In the highest beds of Kerry and Builth (and also on the shelf at Usk) the Loxonema Association occurs in decalcified sandstones which are considered to have been deposited in a moderate energy environment in a shoreface setting. In the western part of the outcrop, in the Kerry and Long Mountain areas, Bradfield (1987) recognizes a return to the olive-grey siltstone facies characteristic of the shelf. Some thin slump bands occur. These facts suggest that the Bishops Castle, Knighton, Radnor Forest and Builth areas were situated in somewhat deeper water than the areas to the west and east. There is still some evidence of the northwards transport of sediment (Bailey & Rees 1973). In the Towy anticlinal area of South Wales, the separate Whitcliffe formations are not easily distinguished. The late Ludfordian strata around Cwm Dwr comprise olive-grey poorly calcareous siltstones with thin layers of sandstone and decalcified shelly limestone bands (Bradfield 1987). A Salopina-Protochonetes Association, depleted in Salopina, is present. Further south, these beds are cut out by the unconformity at the base of the Tilestones. In Pembrokeshire fluviatile deposition continued and is presumably represented by red sandstones of part of the Old Red Sandstone. In North Wales, erosion has removed late Ludfordian strata but pebbles of calcareous siltstone with Whitcliffe fossils occur in the basal conglomerate of the Carboniferous rocks and are presumed to give evidence of a similar facies to that in east-central Wales.
In the Lake District the late Ludfordian includes a large part of the thick Kirby Moor Flags (Shaw 1971). This formation comprises thickly flaggy, sorted siltstones with some slightly calcareous beds and lenses of fossils. A shallow water Salopina-Protochonetes Association occurs but with many more gastropods than usual. The sediments were derived from the northwest. As with Wales, the Lake District basin had been largely infilled (although there was still tectonic subsidence) to produce a shallow subtidal moderate-energy environment. In Ireland, it is postulated that the central region was emergent and being eroded, with rivers providing sediment for the red fluviatile sandstones of the Dingle Group in Kerry. JDL
S6a,b, $7: Northern British Isles: Liandovery (Rhuddanian, Aeronian, Telychian) The palinspastic model used in maps of the northern British Isles, for the Ordovician Period, was developed by J. K. Ingham and his aid in applying it to the Silurian Period is gratefully acknowledged. The timing of the closure of the Iapetus Ocean, the cessation of northwestward subduction and of strike-slip on elemental boundaries are controversial. The maps subscribe to the view that Iapetus continued closing, by northwestward subduction, into the Llandovery Epoch, although during the epoch the ocean may have become little more than an ensialic residual trough. The type of accretionary prism which could have resulted is depicted schematically in Fig. 5. That the two continents Laurentia in the north and Gondwana in the south had been on a collision course for much of the early Palaeozoic and were still separated by this trough in the early Silurian are probably the two least disputed factors of the palaeogeography SE __
downslope movement
.r
NW
i,"
"xr
-...:-.. ;.:~/
Fig. 5. Evolution of Southern Uplands accretionary wedges: accretion stage with southeast-directed thrusting (after Needham & Knipe 1986). One reason for supposing that a collision of the two continental forelands had occurred earlier, in late Caradoc time, is the lack of direct evidence of arc-type volcanicity in either margin after the Caradoc. Bluck (1983) believes there to be indirect evidence for Llandovery activity on the now concealed Cockburnland and distal arc-type volcanism is evinced by the numerous metabentonites in the Moffat Shale, ranging in age from the late Ordovician to the maximus Subzone (early Telychian) (Merriman & Roberts 1990). However, the inference has been drawn that subduction had ceased (Murphy & Hutton 1986). Murphy and Hutton claim that the remnant, or 'successor', basin (Fig. 7) was then bridged merely by clastic infill in the Llandovery. The older view (Mitchell & McKerrow 1975) holds that the Iapetus Ocean, floored by oceanic crust, continued to separate the two continents by more than 1000km until late Silurian times (McKerrow & Soper 1987); from Ordovician to Silurian time northwestward subduction continued while an Ordovician-Silurian thrust-sliced sedimentary prism was incrementally accreted to the Laurentian margin. Although volcanicity in North Wales, southeast Ireland and the Lake District implies that southeastward directed subduction of oceanic crust, beneath northern Gondwana, continued until late Caradoc time there is no documented evidence for the consequential accretionary sedimentary prism on the Avalonian foreland. It is supposed therefore that sometime during the Silurian any such prism underthrust the Laurentian foreland (e.g. Leggett et al. 1983; Stone et al. 1987), or was strike-slipped away.
Southern Uplands / L o n g f o r d - D o w n Z o n e Evidence of the palaeogeography of the Laurentian continental margin resides in northward-dipping southward thrust slices of sedimentary rocks which comprise the Southern Uplands in Scotland and the Longford-Down tract in Ireland. Two different views have been expounded. One adopts the requisites of the accretionary prism outlined by Leggett et al. (1979) with a fore-arc trench, as described by Kelling et aL (1987), being dominated by a variety of depositional systems, mainly fan-type turbidites which had sources to the northwest in arc terrane, already accreted arc-apron sediments, and a continental source. Upon reaching the trench these sediments, in Llandovery times at least, were diverted to the southwest and formed coaxial turbiditic wedges. The early Llandovery sediments of each wedge are recognizably different from the next, e.g. the Kilfillan Formation (KF) and Money Head Formation (MHF) of Fig. 6A, suggesting that the trench was not 'a single unconstrained basin', but divided by structural barriers. Sole structures including flute-casts are common. These coarse detrital sediments had an oceanward limit, possibly confined by an outer trench high, so that hemipelagic anoxic muds were accumulating to the southeast: the Moffat Shales. Of the fault-bound sequences in each of the accretionary slices recognized by Leggett et al. (1979, fig. 2), that now
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
48
SILURIAN
Fig. 6. Schematic diagram depicting the palaeogeographical and geotectonic evolution of the SW Southern Uplands during the early Llandovery (as illustrated by the Rhinns of Galloway and Luce Bay sections). CM, Chippermore Member; CFM, Cairnie Finnart Member; DPM, Daw Point Member; FBF, Float Bay Member; GF, Garheugh Formation (with a and b Formations); GPF, Grennan Point Formation; KF, Kilfillan Formation; MHF; Money Head Formation; SBb, Striking Bight beds. 1, 2, a, b, etc., represent earlier accreted tectono-stratigraphic tracts (from Kelling et al. 1987, fig. 13).
the oceanic area was accumulating carbonaceous graptolitic muds and continued to do so for a further c. 10 Ma. Leggett (1987) claimed that this supported the existence of the oceanic plate, and thus Iapetus Ocean, until the Wenlock, but there seems no reason why such deposition could not have carried on over the continental crust of Murphy & Hutton's 'successor basin' (Fig. 7) in its early phase (Murphy & Hutton 1986). However, whether because of a northwestward moving oceanic plate or by southeastward facies progradation (Griffith & Wilson 1982, fig. 4), greywacke appears in the Irish sequences, in sedgwickii Biozone at Coalpit Bay, in turriculatus Biozone at Tomgraney (Rickards & Archer 1969) and possibly in the griestoniensis Biozone in the western end of the Slieve Bernagh Inlier (Plegg, in Leggett et al. 1979). Greywacke and minor flows of basic lava occur at Slieve Aughty (Emo & Smith 1978), but their ages are not known precisely. The alternative views of Murphy & Hutton (1986) and Stone et al. (1987) are not represented in the accompanying maps. These can account for the same thrust-imbricate, Ordovician and Silurian submarine fan sequences of the Southern Uplands (and the Longford-Down Zone), with their southeastward overlap of pelagic mud deposits (Moffat Shales), in terms of thinskin thrusting of the deposits of a back-arc basin; a basin which had existed largely undeformed through much of the Ordovician into the early Llandovery (Stone et al. 1987, fig. 3; Fig. 8). They cite the andesite-rich composition of some of the greywacke fans and their derivation from the south as evidence for the existence of a now concealed Ordovician/early Llandovery active volcanic arc to the south, with a fore-arc basin beyond. Kelley & Bluck (1989) have more recently suggested that this detritus is of Cambrian (or earlier) age.
Fig. 8. Back-arc basin model for Southern Uplands (from Stone et aL 1987, fig. 3). Oblique collision of the Laurentian and Avalonian foreland margins during the mid-late Llandovery caused cessation of northward subduction at an island arc and produced instead northward underthrusting with the adjacent back arc basin. Eventually the postulated arc terrane underthrust the back-arc basin and the Avalonian foreland underthrust the Laurentian foreland basin. The contents of the back-arc basin rose as a thin-skin, southward propagating, thrust stack; the Moffat Shales acted as the decollement zone and the stack thrust southeastwards, ramping over the arc terrane during latest Telychian times (Fig. 9).
Fig. 7. Model for structure of Southern Uplands from Murphy & Hutton (1986. fig. 3). CB = central belt; SB = southern belt; E - E = approximate level of erosion. forming Tract 4 (IV) reached the turbiditic greywacke environment of the trench in early Llandovery time (e.g. the Pyroxenous Group of Talla and the Money Head and Kilfillan formations; see Fig. 6A). Keiling et al. (1987, p. 803) depict the trench environment at this time (Fig. 6). Sequences in the more southeasterly tracts show that in persculptus/acuminatus times dark grey graptotitic muds were being deposited in the oceanic regions beyond the trench. It was not until early turriculatus times and griestoniensis times that the trench-parallel strips of this ocean floor which now comprise Tracts 5 and 6 respectively of the prism reached the trench environment of greywacke (Leggett et al. 1979). Though less clearly exposed, the sequences in the Longford-Down Zone of Ireland indicate a similar pattern. In the northern tracts, e.g. just southeast of Orlock Bridge on the east coast, the oldest Silurian rocks, probably of acuminatus age, are arenaceous greywackes of the Portavo Formation (Griffith & Wilson 1982, p. 13). Further to the WSW, e.g. at Strokestown, persculptus-acuminatus deposition was of turbidites and massflow volcaniclastic conglomerates while in acuminatus times basalts were extruded (Leggett et al. 1979). These might suggest a trench environment, but without the coarse clastic fans and axial wedges of the central belt of the Southern Uplands of Scotland. Thus it can be argued that those facies were advancing southwestward along the trench slope axis but had only just reached the eastern coast of Ireland by acuminatus times. To the south, as far probably as Tomgraney (Rickards & Archer 1969),
Fig. 9. Sketch section illustrating the southward migration of the Southern Uplands thrust stack and initiation of the foreland basin during the late Llandovery Epoch (from Stone et al. 1987, fig. 5).
Midland Valley of Scotland: south Mayo and north Galway Northeastward of the accretionary prism (or thrust stack of Stone et al. (1987)) lay Laurentian continental crust. On this, in the Llandovery, there was a marine basin forward of an Ordovician volcanic arc, possibly still active, and behind was a back-arc basin, or inter-arc basin of Bluck (1983; Fig. 10). According to Bluck the prism (or 'thrust stack') overrode northwestward much of this Midland Valley terrane in post-Silurian times, thus
49
50
SILURIAN
concealing all but the back-arc basin tract lying through the Midland Valley to south Mayo and north Galway (Fig. 10, ii). Evidence of the concealed terrane has been found in xenoliths (Upton et al. 1983) and the northward-transported pebbles of Wenlock conglomerates in the Midland Valley Silurian inliers (Bluck 1983). Bluck deduced that these were derived fluvially from a catchment which extended at least 70 km southward of the Southern Uplands Fault, a distance commensurate with present trench-arc gaps. This Wenlock 'Cockburnland' (Bluck 1983, pp. 126-7) probably commenced its emergence at the end of the Llandovery as a series of islands.
S8a: Northern British Isles: Wenlock (Sheinwoodian)
Ii - -
- - - MIDLAND
VALLEY
~
HIDDEN
MIDLAND
VALLEY
---
~
SOUTHERN UPLANDS . . . .
arc x'7"k a rc interarc
. . . . . . . .
!
~
1
MIDLAND
VALLEY
fore-arc basin
SOUTHERN
of Llandovery age. The fluvial gravels were overwhelmed eventually by a marine transgression and succeeded by shallow marine sands and gravels with the remains of shells including brachiopods. The Gowlaun Conglomerate, considered by Piper (1972) to represent a deep water base-of-slope deposit, has recently been reinterpreted as a shallow water deposit (Williams & O'Connor 1987). Thus without the need to accommodate a temporary deep water environ, strong comparison can be made with the Midland Valley of Scotland except that in Ireland the Silurian rests partly on the displaced Dalradian block of Connemara. RC
I
accretionary trench
prism
UPLANDS
i
Fig. 10. (i) Interarc basin, missing fore-arc basin (beneath Southern Uplands) and Southern Uplands accretionary prism. (ii) Overthrusting of accretionary prism: 1 = 'igneous conglomerate', 2 = 'quartzite conglomerate', 3 = 'greywacke conglomerate'. 1 and 2 have a source in rock suggested to have been overthrust by the greywackes of the Southern Uplands (from Bluck 1983). The back-arc ('interarc') basin of Llandovery times is witnessed in the rocks of the Midland Valley Silurian inliers including those near Girvan and in the northern Irish inliers of Pomeroy, Lisbellaw, Charlestown and Clew Bay in Mayo and Killary Harbour in north Galway. The earliest Silurian deposits are the persculptus-acuminatus graptolitic muds of Pomeroy. These follow Ashgill deposits without a break and Rrobably represent the deepest parts of this basin. To the east at Craighead, northeast of Girvan (Freshney 1959) a marine transgression over Hirnantian rocks was under way in acuminatus time. Basal deposits of gravel appear to be derived from the Midland Valley arc to the north and were distributed as channel fills. It is necessary therefore to invoke an E-W land-ridge of Midland Valley terrane lying to the north (Bluck 1983) to provide this detritus. Shelly sands predominated through the early Llandovery at Girvan, but in convolutus-early sedgwickii times graptolitic muds interrupted this deposition. Cocks & Toghill (1973) considered these muds to have been deep water deposits comparable with the Moffat Shales, but they probably record merely a slight deepening and concomitant marine transgression of coastal plains, such as was occurring on Avalonia at this time. Shelly muds and sands returned to the area even in the later part ofsedgwickii time with first a Lingula-Eocoelia fauna then Pentamerus and finally CIorinda, before thinly bedded graptolitic sands and muds became established in early turriculatus (maximus) time. These latter may indeed record some deepening of the sea; accommodation of sediment here was certainly rapid. Such sedimentation, considered by Cocks & Toghill to be turbiditic, continued through griestoniensis time when purple, red and green mudstones and sandstones were deposited. Similar conditions at this time are reflected in the other Midland Valley inliers with palaeocurrent directions from the south in the Hagshaw Hills and near Lesmahagow, but in the Pentland Hills, where the Haggis Grit and Conglomerate occurs, palaeocurrents came from the east. The deposition of this 'conglomerate' may have heralded the Wenlock gravels which record fluvial infill from an emergent Cockburnland to the south. Overall it seems unlikely that these Midland Valley reaches of the basin were ever 'deep' in the Llandovery. The deep parts of the basin continued to be around Pomeroy and Lisbellaw where graptolitic muds persisted through sedgwickii and griestoniensis times. At Lisbellaw, Harper & Hartley (1938) recorded conglomerates of mid-Llandovery age and likened them to the Caban Conglomerate of the same age in mid-Wales. Clearly a basin-margin slope existed to the northwest of Lisbellaw and Pomeroy, for at Charlestown, in Co. Mayo, sands and calcareous deposits formed during sedgwickii to crenulata times accompanied by shelly faunas including corals, brachiopods and trilobites. Similai" conditions prevailed in the extreme west around Killary Harbour and Clew Bay where basal gravel deposits ofgriestoniensis age overstepped a Dalradian and Ordovician substrate. In early and mid Llandovery times this area was land and the gravels were interpreted by Piper (1972) as fluvial, with westward followed by southward transport. An extrusive keratophyre was present in places below the gravel, recording volcanicity probably also
Evidence from the rocks themselves for early Wenlock palaeogeography in northern Britain is confined to the Southern Belt of the Southern Uplands of Scotland and its Irish equivalents in the inliers of Slieve Aughty and Slieve Bernagh, to the Midland Valley of Scotland, and to the rather separate western Irish area of south Mayo and north Galway. Otherwise the picture must be fitted with what has gone before and what follows. Relationship with southern Britain remains unknown, though, as there have been suggestions that there is considerable similarity between some of the Silurian rocks in the southern part of the Southern Uplands of Scotland and those in the Lake District of northern England, these areas may have been in close proximity. The Southern Belt of the Southern Uplands contains faulted strips of Wenlock rocks. Those of mid Sheinwoodian age are graded greywackes, only occasionally thick and pebbly, siltstones, mudstones, and graptolitic shales. These Riccarton Beds also yield orthoconic cephalopods. Tool marks are conspicuous among the sole structures and there are also ripples indicating bottom currents at variance with the axial flow of the turbidity currents. In the group of faulted inliers of Slieve Aughty, the Derryfadda and Killanena Formations, found respectively to the north and south, are broadly of this age as indicated by spores and acritarchs (Emo & Smith 1978). The Derryfadda Formation comprises proximal turbidites associated with siltstones and mudstones; they are cut by strike faults. The Killanena Formation is of thinner bedded and finer grained greywackes interbedded with laminated beds. The situation in the complex of inliers of Slieve Bernagh, the Broadford Mountains, and the Arra Mountains is still not fully described in modern terms. Turbidites appear here above the Monograptus crispus Biozone (Telychian) and continue through the Wenlock. Anderson & Oliver (1986) have stressed the importance of the Orlock Bridge-Kingledores Fault, which separates the Northern and Central belts of the Southern Uplands of Scotland and their equivalents in Ireland. Hutton & Murphy (1987) have suggested that the northerly sedimentary source customarily referred to as Cockburnland has been lost at this time by sinistral strike-slip displacement along this fault. Several of the Silurian inliers of the Midland Valley of Scotland (Lesmahagow, Hagshaw Hills, Carmichael, Pentland Hills) show a regressive sequence above marine Llandovery beds. Above the various conglomerates which, except at Lesmahagow, mark the base of the Wenlock, there are arenaceous beds with some red sediments and horizons rich in fossil fish. These coarser Wenlock deposits are seen as alluvial fans derived from a lost source to the southeast. The Louisberg-Clare Island succession in south Mayo shows crossstratified pebbly sandstones, which Phillips (1974) showed to be derived from the northwest. There is evidence of fluvial channels. Separated from the above by faulting, and metamorphosed to lower greenschist facies, the Croagh Patrick sequence is of shallow water, marine sediments. Farther south in the country from Killary Harbour to Lough Mask, the Lettergesh Formation is of Sheinwoodian age and has actually yielded Monograptus riccartonensis at one level. The formation represents the maximum transgression here. Poorly sorted sandstones with conglomerates, breccias, and silty mudstones were deposited from turbidity currents. CHH PRiDOLI The earlier Silurian initiation of progradation of terrestrial sediments across marine areas of the British Isles led to the continually progressive and dominant establishment of non-marine depositional regimes through P~idoli times. By the end of this interval, only present-day areas of southeasternmost England remained open to fully marine influences from the south and southeast, although much of the former Welsh Borderland-Midlands platform was susceptible to short-lived, periodic flooding by the marginal sea until well into the Pfidoli. Alluvial processes of erosion and deposition were persistent to the west and north, leading to the diachronous spread of Old Red Sandstone magnafacies from those general directions. Although the Iapetus Ocean was probably closed well before the Pfidoli Epoch (e.g. Picketing et al. 1988), and there is no evidence of any remaining marine separation between northern and southern Britain at this time, the precise relative positions of the two regions remain uncertain. Sinistral transpressional convergence continued throughout the Pfidoli and later (Soper et al. 1987), so that it is unlikely that there was any direct north-south relationship and continuation of drainage and depositional systems between the two areas that are now adjacent on opposite sides of the sutural zone. Background references to stratigraphical successions of P~idoli age, and their correlation, are given by Bassett et al. (1982), White & Lawson (1989),
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
52
SILURIAN
Cocks et al. (1971), and House et al. (1977). The first two of these publications also give details of environmental developments, especially for the earlier P~idoli, so that only subsequent or additional sources of reference are quoted here. MGB
S8b: Southern British Isles: earliest Pfidoli Across the Welsh Borderland-Midlands platform and into eastern to southcentral Wales the correlation of early P~idoli strata is reasonably well constrained by the presence of an ostracode assemblage that includes Frostiella groenvalliana and species of Londinia and Nodibeyrichia, together with conodonts and thelodonts that are also of limited vertical range. The accompanying presence of relict Ludlow shelly macrofaunas testifies to a continuing marginal marine influence, whilst at the same time the increased occurrence of freshwater fishes, vascular land plants, and spores, accompanied by a sharp fall in the earlier dominance of marine microplankton (e.g. acritarchs) is indicative of the pulsatory encroachment of closely adjacent land areas. The transition from fully marine through quasi-marine facies during this interval went hand in hand with the inversion of the depositional basin, which had commenced similarly in earlier Silurian times and was to reach a climax in Acadian shortening during the early-mid Devonian. Following latest Ludlow regressive shallowing, the Ludlow Bone Bed Member at the base of the Downton Group in the Welsh Borderland represents an initial brief and temporary interval of renewed shallow marine transgression, in which the bone accumulations formed lag concentrates distributed widely across retreating strands. Local evidence of hummocky cross stratification in immediately overlying Downton beds suggests that storm processes were active in shoreface environments showing little or no evidence of tidal influence, in water depths of only a few metres (Smith & Ainsworth 1989); reduction in salinity is also inferred from the accompanying restriction of faunas. Slightly higher beds in the Downton Castle Sandstone Formation across the platform are dominantly fine sandstones with interbedded siltstones and mudstones; the facies are probably subtidal, passing yet higher into thicker sand facies with structures indicative of vigorous wave action in intertidal and inshore zones (Allen 1985, p. 92). These complexes were deposited as early Pfidoli sand shoals that occupied a minimum area of some t2 000 km 2, and the diversity of transport directions may point to more than one source area. Local easterly coarsening in the Midlands (Gornal Sandstone facies) suggests near shore environments with derivation from the east or southeast. Other sources on the platform suggest westerly transport vectors, possibly in part from local highs in the remnant Welsh trough (Allen 1985, fig. 3), but probably too from yet farther to the west and northwest where direct evidence of P~idoli beds is lacking; however, there is no reason to doubt that areas such as the Irish Sea platform were positive suppliers of sediment at this time. Although impersistent bone beds are present locally along the eastern margins of the Welsh trough (e.g. Holland & Williams 1985) they are absent in the central axis where deposition was apparently continuous from the Ludlow to the Downton and where non-marine facies did not become dominant finally until slightly later than on the bounding platform areas. In immediately post-groenvalliana Biozone times, the sea in the Welsh area had probably shrunk to a small central remnant, with tidal sand shoals and extensive mud fiats to the east. At the southern margin of the Welsh trough, early Pfidoli highly micaceous, coarse shelly arenites of the Long Quarry Formation (Tilestones) represent high energy, open shoreface and barrier sands derived from alluvial plains developed on the planated metamorphic basement of Pretannia to the south (Cope & Bassett 1987). Old Red Sandstone facies of southwest Wales (low-middle Milford Haven Group) accumulated on coastal mud fiats flooded periodically by the sea, but at times scoured by rivers and with local alluvial fans fed from either the south or southeast; reworking of older Silurian sources and of basement complexes were both involved (Allen & Williams 1978). Yet farther to the west, in Ireland, there is only limited evidence for P~idoli beds, but terrestrial red bed facies fairly certainly persisted throughout the epoch (e.g. Holland 1979; Doran et al. 1973). The thick Dingle Group of southwest Ireland, comprising coarse arenites and rudites of alluvial origins, is apparently conformable on Ludlow beds (Holland 1987). Direct marine connections northwards from the Wales-Welsh Borderland area probably persisted in earliest P~idoli times to the Lake District, where the lower Scout Hill Flags bear a groenvalliana Biozone fauna in siltstones, mudstones and fine sandstones derived in part from the north. Similar definitive evidence of age is lacking in areas to the east and southeast of the Midlands platform, although limited data from the subsurface suggests that marginal marine sedimentation persisted there as shoals and bars, with more open marine environments beyond, across southeasternmost England. MGB
$9: All British Isles: mid Pfidoli Biostratigraphical data for identifying younger P~'idoli strata across the British Isles are sparse, although limited evidence is available from fish faunas and there is growing potential from the use of sporomorphs (e.g. Richardson & McGregor 1986; Richardson & Edwards 1989). Mid Pfidoli equivalents fall within sporomorph Biozone A (see Richardson & Edwards 1989, fig. 152), taken here within the Ledbury Formation of the Downton
Group and its correlatives, somewhat below the level of the Townsend Tuff; the latter horizon forms a useful lithostratigraphical datum across much of southern Wales and the Welsh Borderland (Allen & Williams 1981, 1982), lying within the upper part of the Pfidoli rather than at the base of the Devonian (Bassett 1984, p. 301; cf. Allen & Williams 1981). Periodic marine incursions continued to influence sedimentation across much of southern England and Wales throughout mid P[idoli times. Sporadic accumulations of bone bed strands that persist well into the Downton Group (e.g. Temeside Bone Bed; Antia 1981) are absent in the Ledbury Formation, where sediments are dominated by thick mudstones with fine siltstones and sandstones. Lingulacean brachiopods and bivalve molluscs (e.g. Allen 1973) reflect the marginal marine episodes of flooding across extensive, featureless mudflats that were partly intertidal and partly river influenced (Allen 1985). Drifted plant debris and articulated vertebrates become increasingly common upwards through the sequence. Widespread developments of pedogenic carbonates (calcretes) indicate the formation of stable geomorphic surfaces that were starved of fluvial input for periods of some 104 years (Allen 1974), and they also suggest a relatively hot climate with comparatively low seasonal rainfall. Deposition across the alluvial plains was cyclical, with repeated incursions of gravels both from major stream systems derived from distant uplands and also from minor systems of ephemeral drainage developed on slightly elevated interfluves between the principal rivers (Allen 1964; Allen & Williams 1979). These later Downton Group sediments thus record a considerable enlargement of quasi-marine to non-marine environments established in earlier Pfidoli times (Allen 1985, p. 94). To the north of the former Welsh Basin there is no evidence of continued marine connection northwards. The Traeth Bach Beds of Anglesey, possibly of mid Pfidoli age, include calcretes within arenites and rudites that may have been derived in part from local fault scarps to the southeast (Allen 1965), although the regional palaeoslope and drainage pattern was fairly certainly persistently from the northwest. Across southwest Wales the Pfidoli alluvial systems developed partly in a major west-east aligned valley complex (Allen & Williams 1982; Allen 1985, fig. 3), with sediment input from both the north and south. The whole of Ireland was probably by now the site of continuous alluvial red bed facies and erosive systems, with deposition of the Dingle Group in the southwest continuing throughout this interval (Holland 1979; Doran et al. 1973; Horne 1974). The only evidence of more open marine conditions in the post-early Pfidoli of Britain is across southern England, where micaceous sandstones and siltstones in the subsurface at Little Missenden formed probably as subtidal sand sheets with a restricted shelly fauna. Similar sediments occur at Lakenheath and Soham, although there are no faunal/floral data to confirm the age. As in the earlier P?idoli, an open marine connection to the southeast is indicated. By the end of Pfidoli times the marginal marine influence across the former Welsh trough and its bounding platform areas had ceased, and instead fluvial sedimentation then became ubiquitous. In northern Britain, biostratigraphical data that permit precise recognition of Pfidoli ages in successions are even more sparse than to the south. However, recent geochronological studies (Thirlwall 1988) have radically changed the stratigraphical assignment of some of the Old Red Sandstone deposits in Scotland; a good deal of the red terrestrial sediment assumed previously to be of Devonian age is now known to be Silurian, and some basins active in mid P~'idoli times were initiated late in the Wenlock. Whilst most of the Grampian block has cooling ages of >425 Ma, rocks alongside the Great Glen Fault have cooling ages going down to 385 Ma, and here as well as in the northern Highlands there are many cooling ages between 420 and 400 Ma, suggesting that the mid Pi'idoli was a time of substantial uplift in both these areas. In northern Scotland there was uplift along the Moine Thrust in early Ludlow times, so that most of the ground to the east of that structure was then undergoing elevation and must have provided a substantial source of sediment for any basins that may have been forming contemporaneously. In addition, numerous granitoid intrusions are recorded from the Dalradian block to the southeast of the steep belt. These are thought to be high-level plutons that may have been associated with lavas. One plutonic complex occurs within the steep belt and near an area of uplift at Findhorn, and far in the southwest the Ross of Mull granite is of Pi'idoli age. There are a number of intrusions within the Midland Valley, including Distinkhorn, Tinto, and diorite intrusions in the Ocl:fil Hills. Contemporaneous lavas are also dated from the Pentland Hills and from Fife, so the Midland Valley is seen as a magmatic province with lavas > 3 km thick. Although there are lava flows over much of the Midland Valley, two major centres are developed, in the OchiI-Sidlaw Hills and in the Pentlands; these lavas, accompanied by agglomerates and tufts, range from rhyolites, dacites and andesites to basalts. Intrusive rocks also occur along the northern edge of the Southern Uplands at Carsphairn, Cockburn Law, and Priestlaw, although it is not known whether they were accompanied by lavas. The earliest sedimentary basin in the north is at the southwest end of the Great Glen Fault near Oban. This basin originated in the late Wenlock with an early fill of coarse clastic sediment. By mid Pfidoli times the whole basin was covered with lavas of the Lorne plateau, although these are mainly or wholly of Ludlow age. In the Midland Valley, palynological data from the Stonehaven Group indicate that the beds are probably of late Wenlock age (Marshall 1989) and not within the Pfidoli or younger as assumed previously. There is probably a
53
54
SILURIAN
stratigraphical break between these beds and overlying units of the Lower Old Red Sandstone (e.g. D u n n o t t a r and Crawton groups), which may well span all or part of the Pfidoli Series, although no precise dating is yet available. These post-Stonehaven units contain boulder-bearing conglomerates tfiat were discharged from the northeast. The conglomerates contain clasts of quartzite, greywacke, a variety of granites, and other intrusives-but almost always present are the abundant and varied lava clasts. The quartzites and some of the porphyries are well rounded, the former being polycyclic. Lavas occur within the sequence, one being associated with slumping and possible contemporaneous faulting; others form useful stratigraphical markers, as at Crawton. Haughton (1988) has shown that some conglomerates of this age were derived from the southeast and are typified by the presence of a b u n d a n t greywacke clasts and granites, whose age and chemistry differs from those derived from the northwest and northeast; this indicates that the source was a previously unsuspected greywacke pile (of Ordovician age) in the centre of the Midland Valley. Elsewhere in the northern region of the Midland Valley there are abundant lavas and lava-bearing conglomerates. Some of these overlie the Lintrathen ignimbrite and could well be of mid Pfidoli age. Intrusions such as those at Comrie may have been associated with lavas at higher structural levels. Basins were also developing in the southern Midland Valley at this time. The distinctive Greywacke Conglomerate, with clasts dominated by greywacke, is cut by felsites and also contains felsite clasts; both felsites resemble that at Tinto, so the conglomerate is thought to be contemporaneous. The greywacke clasts do not have a composition compatible with a source in the Southern Uplands, and often show a dispersal direction towards the southwest; their provenance remains uncertain. A prominent conglomerate succession developed in the eastern Southern Uplands is cut by lamprophyre dykes of 395 Ma or older. The age of the conglomerates is uncertain, but some may fall within the mid P[idoli interval; they are of local derivation and are dispersed towards the northwest. MGB, BJB Acknowledgements. The assistance of the following in compilation of this section is gratefully acknowledged: S. G. D. Campbell, J. R. L. Allen, D. R. Atkins, R. J. Bailey, K. E. S. Bradfield, L. Cherns, L. R. M. Cocks, J. R. Davies, R. Evans, C. J. N. Fletcher, B. A. Hains, M. Leng, L. M. Ellis, R. Marsh, A. J. Reedman, R. B. Rickards, A. W. A. Rushton, M. Scott, J. T. Temple, E. V. Tucker, S. P. Tunnicliff, P. T. Warren, D. E. White, N. J. Woodcock, J. Zalasiewicz, S. G. Molyneux and M. F. Howells.
R e f e r e n c e s
References not listed here are cited in Cocks et al. (1971) ALLEN, J. R. L. 1962. Intraformational conglomerates and scoured surfaces in the Lower Old Red Sandstone of the Anglo-Welsh cuvette. Liverpool and Manchester Geological Journal 3, 1-20. - 1964. Studies in fluviatile sedimentation: six cyclothems from the Lower Old Red Sandstone, Anglo-Welsh basin. Sedimentology, 3, 163-198. - 1965. The sedimentation and palaeogeography of the Old Red Sandstone of Anglesey, North Wales. Proceedings of the Yorkshire Geological Society, 35, 139-185. - - 1973. A new find of bivalve molluscs in the uppermost Downtonian (Lower Old Red Sandstone) of Lydney, Gloucestershire. Proceedings of the Geologists" Association, 84, 27-30. - 1974. Studies in fluviatile sedimentation: implications of pedogenic carbonate units, Lower Old Red Sandstone, Anglo-Welsh outcrop. Geological Journal 9, 181-208. - 1985. Marine to freshwater: the sedimentology of the interrupted environmental transition (Ludlow-Siegenian) in the Anglo-Welsh region. In: CnALONER,W. G. & LAWSON,J. D. (eds) Evolution and environment in the late Silurian and early Devonian. Philosophical Transactions of the Royal Society of London, B309, 85104. -& WILLIAMS, B. P. 1978. The sequence of the earlier Lower Old Red Sandstone (Siluro-Devonian), north of Milford Haven, southwest Dyfed (Wales). Geological Journal 13, 113-136. -- & -1979. Interfluvial drainage on Siluro-Devonian alluvial plains in Wales and the Welsh Borders. Journal of the Geological Society, London, 136, 361-366. -& -1981. Sedimentology and stratigraphy of the Townsend Tuff Bed (Lower Old Red Sandstone) in South Wales and the Welsh Borders. Journal of the Geological Society, London, 138, 15-29. -- & -1982. The architecture of an alluvial suite: rocks between the Townsend Tuff and Pickard Bay Tuff beds (early Devonian), southwest Wales. Philosophical Transactions of the Royal Society of London, B297, 51-89. ANDERSON, T. B. & OLIVER,G. J. H. 1986. The Orlock Bridge Fault: a major Late Caledonian sinistral fault in the Southern Uplands terrane, British Isles. Transactions of the Royal Society of Edinburgh: Earth Sciences, 77, 203-222. ANT1A, n. D. J. 1980. Palaeontological and Sedimentological Studies on the Upper
Silurian Ludlow-Downton Series Transition in the Welsh Borderlands and some Phanerozoic Bone-Beds. PhD thesis, University of Glasgow. 1981. The Temeside Bone-Bed and associated sediments from Wales and the Welsh Borderland. Mercian Geologist, 8, 163-215. ARTHURTON,R. S., JOHNSON, E. W. & MUNDY, D. J. C. 1988. The Geology of the Country around Settle. Memoirs of the British Geological Survey. Sheet 60 (England & Wales) HMSO. ATKINS,D. R. 1979. Faunal Distribution and Depositional Em,ironments in the Lower Bringewoodian ( Ludlovian) of Wales and the Welsh Borders. PhD thesis, University of Glasgow. BAILEY, R. J. 1969. Ludlovian sedimentation in south central Wales. In: WOOD, A. (ed.) The Precambrian and Lower Palaeozoic Rocks of Wales. University of Wales Press, Cardiff, 283-304. - - & REES, A. I. 1973. A magnetic fabric study of late Whitcliflian (u. Silurian) siltstones from the Welsh Borderland. Geological Journal 8, 179-188. -
BAKER,J. W. & HUGHES,C. P. 1979. Summer (1973) Field Meeting in Central Wales, 31 August to 7 September 1973. Proceedings of the Geologists" Association, 90, 65-79. BASSETT,M. G. 1974. Review of the stratigraphy of the WeDlock Series in the Welsh borderland and South Wales. Palaeontology, 17, 745-777. - 1984. Lower Palaeozoic Wales--a review of studies in the past 25 years. Proceedings of the Geologists'Association, 95, 291-31 I. - 1989. The WeDlock Series in the WeDlock area. In: HOLLAND,C. H. & BASSETT, M. G. (eds) A Global Standard for the Silurian System, National Museum of Wales, Geological Series No. 9, Cardiff, 51-73. - - , COCKS, L. R. M., HOLLAND,C. H., RtCKARDS,R. B. & WARREN,P. T. 1975. The o,pe WeDlock Series. Report of the Institute of Geological Sciences, No. 75/ 13. - - , LAWSON,J. D. & WHITE,D. E. 1982. The Downton Series as the fourth Series of the Silurian System. Lethaia, 15, 1-24. BENTON, M. J. & GRAY, D. I. 1981. Lower Silurian distal shelf storm-induced turbidites in the Welsh Borders: sediments, tool marks and trace fossils. Journal of the Geological Society, London, 138, 675-694. BLVCK, B. J. 1983. Role of the Midland Valley of Scotland in the Caledonian orogeny. Transactions of the Royal Society of Edinburgh: Earth Sciences, 74, 119-136.
BOTT, M. H. P. 1967. Geophysical investigations of the northern Pennine basement rocks. Proceedings of the Yorkshire Geological Society, 36, 139-168. BRADFIELD, K. E. S. 1987. Late Ludlovian and early Pfldoli palaeoenvironments of mid- Wales and the Welsh Borderland. PhD thesis, University of London. BRENCHLEY, P. 1984. Late Ordovician extinctions and their relationship to the Gondwana glaciation. In: BRENCHLEY, P. (ed.) Fossils and Climate. Wiley, Chichester. BRIDEN,J. C., MORRIS,W. A. & PIPER,J. D. A. 1973. Paraeomagnetic Studies in the British Caledonides--Vl. Regional and Global Implications. Geophysical Journal of the Royal Astronomical Society, 34, 107-134. BRIDGES, P. H. 1975. The transgression of a hard substrate shelf: the Llandovery (lower Silurian) of the Welsh Borderland. Journal of Sedimentary Petrology, 45, 79-94. - - 1976. Lower Silurian transgressive barrier islands, southwest Wales. SedimentoIogy, 23, 347-362. BR0Cg, P. M., 1972. Stratigraphy and sedimentology of the Lower Palaeozoic greywacke formations in counties Kildare and west Wicklow. Proceedings of the Royal Irish Academy, ii72, 25-53. - - , COLTHURSr,J. R. J., FELLY,M., GARDINER,P. R. R., PeNNEr, S. R., REEVES, T. J., SHANNON, P. M., SMITH, D. G. & VANOUESTAINE,M. 1979. South-east Ireland: Lower Palaeozoic stratigraphy and depositional history. In: HAgltlS, A. L., HOLLAND,C. H. & LEAKE,B. E. (eds) Caledonides of the British Isles-Reviewed. Geological Society, London, Special Publication, 8, 533-544. BURGESS, I. C. & HOLLIDAY,D. W. 1979. Geology of the country around Broughunder-Stainmore. Memoirs of the Geological Survey of Great Britain, Sheet 31 (England and Wales). HMSO, London. CAMPBELL,S. D. G., 1984. Aspects of dynamic stratigraphy (Caradoc-Ashgill) in the northern part of the Welsh marginal basin. In: Abstracts of current research in Wales. Proceedings of the Geologists' Association, 95, 390-391. CAVE, R. & HAINS, B. A. 1986. Geology of the country between Aberystwyth and Machynlleth. Memoirs of the British Geological Survey, Sheet 163. (England & Wales). HMSO. London. CHEgNS, L. 1979. The environmental significance of Lingula in the Ludlow Series of the Welsh Borderland and Wales. Lethaia, 12, 35-46. - 1980. Hardgrounds in the Lower Leintwardine Beds (Silurian) of the Welsh Borderland. Geological Magazine 117, 311-326. - 1988. Faunal and facies dynamics in the Upper Silurian of the Anglo-Welsh basin. Palaeontology, 31, 451-502. CocKs, L. R. M. & TOGHILL,P., 1973. The biostratigraphy of the Silurian rocks of the Girvan District, Scotland. Journal of the Geological Society, London, 129, 209-243. , HOLLAND, C. H., RICKARDS,R. B. & STRACHAN, I. 1971. A correlation of Silurian rocks in the British Isles. Journal of the Geological Society, London, 127, 103-136. [Also as Geological Society of London, Special Report 1]. , WOODCocK,N. H., RICKARDS,R. B., TEMPLE,J. T. & LANE, P. D. 1984. The Llandovery Series of the Type Area. Bulletin of the British Museum of Natural History (Geology), 38, 131-182, figs 1-70. COPE, J . C . W . & BASSET'r,M. G. 1987. Sediment sources and Palaeozoic history of the Bristol Channel area. Proceedings of the Geologists'Association, 98, 315-330. COPE, R. N. 1959. The Silurian rocks of the Devilsbit Mountain District, County Tipperary. Proceedings of the Royal Irish Academy, B60, 217-242. CROWLEY, S. F. 1985. Lithostratigraphy of the Peel Sandstones, Isle of Man. Mercian Geologist, 10, 73-76. CUMMINS, W. A. 1957. The Denbigh Grits; WeDlock Greywackes in Wales. Geological Magazine, 94, 433--451. - 1959. The Nantglyn Flags; Mid-Salopian basin facies in Wales. Liverpool and Manchester Geological Journal, 2, 159-167. DOPa~N, R. J. P. 1974. The Silurian rocks of the Devilsbit Mountain district, County Tipperary. Proceedings of the Royal Irish Academy, !i74, 193-202. - - , HOLLAND;C. H. & JACKSON,A. A. 1973. The sub-Old Red Sandstone surface in southern Ireland. Proceedings of the Royal Irish Academy, B73, 109-128. DREW, H. & SLATER, I. L. 1910. Notes on the geology of the district around Llansawel (Carmarthenshire). Quarterly Journal of the Geological Society of London, 66, 402-19. EMO, G. T. & SMITH,D. G. 1978. Palynological evidence for the age of the Lower Palaeozoic rocks of Slieve Aughty, Counties Clare and Galway. Proceedings of the Royal Irish Academy, !i78, 281-292. FRESHNEV, E. C. 1959. An extension of the Silurian Succession in the Craighead Inlier, Girvan. Transactions of the Geological Society of Glasgow, 24, 27-31. FUgNESS, R. R. 1965. The petrography and provenance of the Coniston Grits east of the Lune Valley, Westmorland. Geological Magazine, 102, 252-260. GRIFEITH,A. E. & WILSON,H. E. 1982. Geology of the country around Carrickfergus and Bangor. Memoirs of the Geological Survey of Northern Ireland, Sheet 29. HMSO. London. HAUGnTON,P. D. W. 1988. A cryptic Caledonian flysch terrane in Scotland. Journal of the Geological Society, London, 145, 685-703. HOLLAND,C. H. 1979. Augmentation and decay of the Old Red Sandstone continent: evidence from Ireland. Palaeogeography, Palaeoclimatology, Palaeoecology, 27, 59-66. -1981. Silurian. In: HOLLAND, C. H. (ed.) A Geology of Ireland. Scottish Academic Press, Edinburgh, 65-81. -1984. Steps to a standard Silurian. In: Stratigraphy. Proceedings of the 27th International Geological Congress, Moscow, 1, 127-156. VNU Science Press, Utrecht.
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
56
SILURIAN
1987. Stratigraphical and structural relationships of the Dingle Group (Silurian), County Kerry, Ireland. Geological Magazine, 124, 33-42. 1989a. Principles, history, and classification. In: HOLLAND,C. H. & BASSETT, M. G. (eds) A global standard for the Silurian System. National Museum of Wales, Geological Series No. 9, Cardiff, 7-26. -1989b. Silurian. In: DUFF,P. McL. D. & SMITH,A. J. (eds) Geology of England and Wales. Geological Society, London (in press). 1988. The fossiliferous Silurian rocks of the Dunquin inlier, Dingle Peninsula, County Kerry, Ireland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 79, 347-360. - - , FEEHAN,J. & WILLIAMS,E. M. 1988. The Wenlock rocks of the Cratloe Hills, County Clare. Irish Journal of Earth Sciences, 9, 61-69. -& BASSETT,M. G. (eds) t989. A global standard for the Silurian. National Museum of Wales, Geological Series No. 9, Cardiff. & LAWSON,J. D. 1963. Facies patterns in the Ludlovian of Wales and the Welsh Borderland. Liverpool and Manchester Geological Journal, 3, 269-288. -& WILLIAMS, E. M. 1985. The Ludlow-Downton transition at Kington, Herefordshire. Geological Journal, 20, 31-41. HORNE, R. R. 1974. The lithostratigraphy of the late Silurian to early Carboniferous of the Dingle Peninsula, Co. Kerry. Bulletin of the Geological Survey of Ireland, l, 395-428. HOUSE, M. R., RICHARDSON,J. B., CHALONER,W. G., ALLEN, J. R. L., HOLLAND, C. H. & WESTOLL, T. S. 1977. A Correlation of Devonian Rocks in the British Isles. Geological Society, London, Special Report, 8. HUTTON, D. H. W. & MURPHY, F. C. 1987. The Silurian of the Southern Uplands and Ireland as a successor basin to the end-Ordovician closure of Iapetus. Journal of the Geological Society, London, 144, 765-772. INGHAM,J. K. & RICKARDS,R. B. 1974. Lower Palaeozoic rocks. In: RAYNER,D. H. & HEMMINGWAY,J. E. (eds) The geology and Mineral Resources of Yorkshire. Occasional Publication of the Yorkshire Geological Society, 2, 29-44. & WRIGHT,A. D. 1972. In: WILLIAMS,A., STRACHAN,I., BASSETT,O. A., DEAN, W. T., INGHAM, J. K., WRIGHT, A. D. & WHITTINGTON,H. B. 1972. A Correlation of Ordovician Rocks in the British Isles. Geological Society, London. Special Report, 3. JACKSON, J. & MCKENZIE, D. 1983. The geometrical evolution of normal fault systems. Journal of Structural Geology, 5. 471-482. KELLEY, S. & BLUCK,B. J. 1989. Detrital mineral ages from the Southern Uplands using 4~ laser probe. Journal of the Geological Society, London, 146, 401-403. KEELING, G., DAVIES,P. & HOLROYD.J. 1987. Style, scale and significance of sand bodies in the Northern and Central Belts, south-west Southern Uplands. Journal of the Geological Society, London, 144, 787-805. -& WOLLANDS, M. A. 1969. The stratigraphy and sedimentation of the Llandoverian rocks of the Rhyader district In: WOOD, A. (ed.) The Precambrian and lower Palaeozoic Rocks of Wales, University of Wales Press, Cardiff, 255-282. LEGGETT,J. K. 1987. The Southern Uplands as an accreting prism. Journal of the Geological Society, London, 144, 737-752. , MCKERROW,W. S., MORRIS,J. H., OLIVER,G. J. H. & PHILLIPS,W. E. A. 1979. The north-western margin of the Iapetus Ocean. In: HARRIS,A. L., HOLLAND,C. H. & LEAKE, B. E. (eds) The Caledonides of the British Isles--reviewed. Geological Society, London, Special Publication, 8, 499-511. --, -& SOPER, N. J. 1983. A model for the crustal evolution of southern Scotland. Tectonics, 2, 187-210. LOCKLEY,i . G. 1983. Brachiopods from a Lower Palaeozoic mass flow deposit near Llanafan Fawr, Central Wales. Geological Journal, 18, 93-99. MARSHALL, J. E. A. 1989. The palynology of the Stonehaven Group, north east Scotland and its stratigraphical implications. Abstracts, Palaeontological Association Annual Conference, Liverpool, 1989, 13. MERRIMAN, R. J. & ROBERTS, B. 1990. Metabentonites in the Moffat Shale Group, Southern Uplands of Scotland. Geological Magazine, 127, 259-271. MITCHELL, A. H. & MCKERROW, W. S. 1975. Analagous evolution of the Burma Orogen and the Scottish Caledonides. Geological Society of America Bulletin, 86, 305-315. MOHAMAD,A. H. B. 1981. The Petrology and Depositional Environment of the Upper Bringewood Beds of the South-eastern part of the Welsh Borderland. PhD thesis, University of London. MURPHY, F. C. & HUTTON,D. H. W. 1986. Is the Southern Uplands of Scotland really an accretionary prism? Geology 14, 354-357. NEEDHAM,D. T. & KNIPE,R. J. 1986. Accretion- and collision-related deformation in the Southern Uplands accretionary wedge, southwest Scotland. Geology, 14, 303-306. PENNEY, S. R. 1980. The stratigraphy, sedimentology and structure of the Lower Palaeozoic rocks of north County Waterford. Proceedings of the Royal Irish Academy, 1380, 305-333. PHILLIPS, W. E. A. 1974. The stratigraphy, sedimentary environments and palaeogeography of the Silurian strata of Clare Island, Co. Mayo, Ireland. Journal of the Geological Society, London, 130, 19-41. PICKERING, K. P., BASSETT,M. G. & SIVETER, D. J. 1988. Late Ordovician--early Silurian destruction of the Iapetus Ocean: Newfoundland, British Isles and Scandinavia--a discussion. Transactions of the Royal Society of Edinburgh: Earth Sciences, 79, 361-382. -
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PIPER, D. J. W. 1972. Sedimentary environments and palaeogeography of the late Llandovery and earliest Wenlock of north Connemara, Ireland. Journal of the Geological Society, London, 128, 33-51. PRENDERGAST,B. M. 1972. The Silurian and Devonian inlier of Knockshigowna Hill, County Tipperary, Ireland. Scientific Proceedings of the Royal Dublin Society, 4, 201-211. RICHARDSON,J. B. & EDWARDS,D. 1989. Sporomorphs and plant megafossils. In: HOLLAND, C. H. & BASSE'I'r,M. G. (eds). A Global Standard for the Silurian System, National Museum of Wales, Geological Series No. 10, Cardiff, 216-226. & McGREGOR, D. C. 1986. Silurian and Devonian spore zones of the Old Red Sandstone continent and adjacent regions. Bulletin of the Geological Survey of Canada, 364, 1-79. RICKARDS, R. B. 1978. Silurian. In: MOSELEY F. (ed.) The Geology of the Lake District, Yorkshire Geological Society, Occasional Publication 3, 130-145. & ARCrmR,J. B. 1969. The Lower Palaeozoic rocks near Tomgraney, Co. Clare. Scientific Proceedings of the Royal Dublin Society, A3, 219-230. - - , BURNS,V. & ARCHER,J. B. 1973. The Silurian sequence at Balbriggan, Co. Dublin. Proceedings of the Royal Irish Academy B73, 303-316. ROBERTS, R. O. 1929. The geology of the district around Abbey-cwmhir (Radnorshire). Quarterly Journal of the Geological Society, London, 85, 651-676. SELDEN, P. A. & WHITE, D. E. 1983. A new Silurian arthropod from Lesmahagow, Scotland. Special Papers in. Palaeontology, 311,43-49. SHAW, R. W. L. 1971. The faunal stratigraphy of the Kirkby Moor Flags of the type area near Kendal, Westmorland. Geological Journal, 7, 359-380. SIVETER,DAVID& TURNER,S. 1982. A new Silurian microvertebrate assemblage from the Tortworth inlier, Avon, England. AIcheringa, 6, 35-41. SMITH, R. D. A. 1987. The griestoniensis Zone Turbidite System, Welsh Basin. In: LEGGE'rr,J. K. & ZUFFA,G. G. (eds) Marine Clastic Sedimentology. Graham & Trotman, 89-107. & AINSWORTH,R. B. 1989. Hummocky cross-stratification in the Downton of the Welsh borderland. Journal of the Geological Society, London, 146, 897-900. SOPER, N. J., WEBB, B. C. & WOODCOCK,N. H. 1987. Late Caledonian (Acadian) transpression in north-west England: timing geometry and geotectonic significance. Proceedings of the Yorkshire Geological Society, 46, 175-192. STONE,P., FLOYD,J. D., BARNES,R. P. & LINTERN,B. C. 1987. A sequential back-arc and foreland basin thrust duplex model for the Southern Uplands of Scotland. Journal of the Geological Society, London, 144, 753-764. TEMPLE, J. T. 1988. Ordovician-Silurian boundary strata in Wales. Bulletin of the British Museum (Natural History) (Geology), 43, 65-71. THIRLWALL,M. F. 1988. Geochronology of late Caledonian magmatism in northern Britain. Journal of the Geological Society, London, 145, 951-967. TYLER, J. E. & WOODCOCK, N. H. 1987. The Bailey Hill Formation: Ludlow Series turbidites in the Welsh Borderland reinterpreted as distal storm deposits. Geological Journal, 22, 73-86. UPTON, B. J. G., ASPEN, P. & CHAPMAN,N. A. 1983. The upper mantle and deep crust beneath the British Isles: evidence from inclusions in volcanic rocks. Journal of the Geological Society, London, 1410, 105-121. WALMSLEY, V. G. d~. BASSEa'r, M. G. 1976. Biostratigraphy and correlation of the Coralliferous Group and Gray Sandstone Group (Silurian) of Pembrokeshire, Wales. Proceedings of the Geologists' Association, 87, 191-220. WALTON, E. K. 1983. Lower Palaeozoic--stratigraphy. In: CRAIG, G. Y. (ed.) Geology of Scotland, Second Edition. Scottish Academic Press, Edinburgh, 105137. WARREN, P. T., PRICE, D., NUTI, M. J. C. & SMITH, E. G. 1984. Geology of the country around Rhyl and Denbigh. Memoirs of the British Geological Survey, Sheets 95 and 107 (England and Wales). HMSO. WATERS, R. A. & LAWRENCE,D. J. D. 1987. Geology of the South Wales Coalfield, Part llI. The country around Cardiff, 3rd edition. Memoir of the British Geological Survey, Sheet 263 (England and Wales). HMSO. WATKINS, R. 1979. Benthic community organization in the Ludlow Series of the Welsh Borderland. Bulletin of the British Museum (Natural History) (Geology), 31, 175-280. & AITHIE,C. J. 1980. Carbonate shelf environments and faunal communities in the Upper Bringewood Beds of the British Silurian. Palaeogeography, Palaeoclimatology and Palaeoecology, 29, 341-368. WroTE, D. E. & LAWSON,J. D. 1989. The Pfidoli Series in the Welsh Borderland and south-central Wales. In: HOLLAND, C. H. & BASSEYr, M. G. (eds) A Global Standard for the Silurian System. National Museum of Wales, Geological Series No. 9, Cardiff, 131-141. WILLIAMS, D. M. & O'CONNOR, P. D. 1987. Environment of deposition of conglomerates from the Silurian of north Galway, Ireland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 78, 129-132. WILLS, L. J. 1978. A Palaeogeological Map of the Lower Palaeozoic Floor below the -
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cover of Upper Devonian, Carboniferous and later formations with inferred and speculative reconstructions Of Lower Palaeozoic and Precambrian outcrops in adjacent areas. Geological Society, London, Memoir, 8.
Ordovician R. E. B E V I N S ,
B. J . B L U C K ,
P. J . B R E N C H L E Y , R. A. F O R T E Y , & A. W . A. R U S H T O N
Since the publication of the Cambrian and Ordovician correlation charts by the Geological Society (Cowie et al. 1972; Williams et al. 1972) it has become more general practice to include the Tremadoc Series in the Ordovician, and although the precise level at which the base of the Ordovician is to be drawn is not finally decided, the limits of the system are generally understood. The upper limit has now been precisely defined at the base of Parakidograptus acuminatus Biozone, as exposed at Dob's Linn in the central belt of the Southern Uplands in south Scotland, the underlying Glyptograptus persculptus Biozone being relegated to the Hirnantian Stage of the Ashgill Series in the Ordovician System (Cocks & Rickards 1988). Standard international series for the Ordovician are yet to be decided upon but in Britain the series usually employed are those which have been defined and refined over a period of more than a hundred years, based on successions in Wales, the Welsh Borderland and the north of England, namely: Tremadoc, Arenig, Llanvirn, Llandeilo, Caradoc and Ashgill. These have ready application in the Anglo-Welsh area and further afield, particularly in continental Europe and Asia, but the demonstration that the faunas of Scotland and northwestern Ireland have much more in common with those of Laurentia (North America, Greenland and Spitsbergen) makes the direct application of many of these chronostratigraphical terms less relevant than the scheme which has developed for the classification of the North American Ordovician. The latter is still to some extent in a state of flux but the version followed here is that tabulated in the lUGS correlation chart for the United States of America (Ross et al. 1982) and with reference to that produced for the IUGS correlation chart for Canada (Barnes et aL 1981). Our increasing knowledge of conodont and graptolite zonations shows that the Canadian Series (broadly equivalent to the Ibexian Series of Ross et al. 1982) correlates essentially with the Anglo-Welsh Tremadoc plus Arenig series. The Whiterockian Series corresponds to the Llanvirn, Llandeilo and, it appears, a very small part of the Caradoc Series as currently understood, i.e. to the top of the largely Llandeilo Nemagraptus gracilis Biozone. The precise position of the base of the Whiterockian Series with respect to the Arenig/Llanvirn boundary is not yet clear. The much-used term 'Chazyan" is now subsumed into the upper part of the Whiterockian Series. The Mohawkian Series is equivalent to a large part of the Caradoc Series and ranges up to an undefined level in the Dicranograptus clingani Biozone and the Cincinnatian Series is thus equivalent to the upper Caradoc Series plus the Ashgill Series. Stage names of usually regional application have not yet been defined for every series in the Ordovician System, whether in southern Britain or in North America but those which are available are widely used and the reader is referred to the references provided above and others given elsewhere in the Ordovician section of the atlas. For clarity, the practice adopted here is to use the 'standard' British chronostratigraphical scheme, even for the Laurentian parts of what now constitutes Britain and Ireland but also to give the North American terms where they are particularly relevant. By the late Ordovician, i.e. latest Caradoc and Ashgill times, 'Atlantic' faunas, both graptolitic and shelly, had become much more cosmopolitan and precise correlation between northern and southern Britain is a relatively straightforward matter. The global positions of the various terranes which now make up the British Isles are reasonably clear. The close relationship of southern Britain and southeastern Ireland with the Avalon terrane of eastern Newfoundland and other parts of maritime North America in late Precambrian and Cambrian times probably persisted into the Ordovician. It appears that the Avalon terrane occupied fairly high to mid-southern latitudes in the early part of the Ordovician but that by the end of the period had migrated to rather lower latitudes with the progressive closure of the Iapetus Ocean (Cocks & Fortey 1982). The various terranes now recognized within northern Britain and Ireland appear, in the early Ordovician, to have been strung out along an accretionary margin of Laurentia at low southern latitudes, only gradually becoming assembled during early andmid-Palaeozoic times into the configuration we recognize today. JKI
Ola: Northern British Isles: Tremadoc In the foreland region of northwestern Scotland the dolomites, limestones and cherts of the Saiimhor and Sangomore formations of the Durness Group are probably of Tremadoc ( = early Canadian/Ibexian) age (Cowie et al. 1972). To what extent sedimentation continued to the east of the Moine 9 1992The GeologicalSociety
C. P. H U G H E S ,
J . K. I N G H A M
Thrust, over the northern Highlands terrane, is not known as any possible Lower Palaeozoic sequences there, autochthonous or allochthonous, have been removed. In western Newfoundland, platformal sediments of the lower part of the St George Group accumulated on the foreland, whereas the Humber Arm allochthon is typified by the off-platform margin Cow Head Group and the basinal Curling Group (see Barnes et al. 1981) which, together with the Bay of Islands Ophiolite was not emplaced until mid Ordovician times. The belt of Dalradian rocks in the Central Highlands had begun to be uplifted at c. 515 Ma (Dempster 1985) and was probably yielding sediment in Tremadoc times but not to the Midland Valley terrane which must have been far removed from it. Pre-Lower Arenig rocks comprising marie and ultramafic assemblages with conglomerates and breccias are found in the Highland Border Complex at the northern edge of the Midland Valley terrane but they are undated and may be Tremadoc or older. Rushton & Tripp (1979) described a northerly derived undeformed limestone clast from the essentially Llandeilo Benan Conglomerate of the Girvan district which yielded a distinctive early Tremadoc (Gasconadian i.e. early Canadian/Ibexian) shelly fauna. This, together with the widespread distribution of Arenig carbonates in the Highland Border Complex (which forms the foundation to the northern part of the Midland Valley terrane) and as southerly derived clasts in the Old Red Sandstone south of Stonehaven, is suggestive of a carbonate platform occupying much of the northern part of the Midland Valley terrane in Tremadoc to Arenig times (see Bluck et aL 1984; Ingham et al. 1986; Haughton 1988). In western Ireland, the Lough Nafooey Group, a succession dominated by pillow lavas has yielded basal Arenig (or highest Tremador graptolites from a shale horizon near its summit (Dewey et aL 1970; Ryan et al. 1980). This group is confined between the transcurrent Doon Rock and Lough Nafooey faults at the southern margin of the south Mayo Lower Palaeozoic tract and conceivably could have been brought into its present position from a backarc basinal source when the Connemara Dalradian tract was translated sinistrally along the Doon Rock Fault in later Ordovician times. In the Girvan district, an age of 501 Ma for lavas at Mains Hill (Thirtwall & Bluck 1984) suggests an early development of the Ballantrae Complex. Metapyroxenite with an age of c. 505 Ma (Hamilton et al. 1984) also indicates that movements were occurring at deep crustal levels within the ophiolite pile. Metamorphic micas with Tremadoc ages are also found in the Ordovician greywackes of the Southern Uplands and Kelley & Bluck (1989) believe these to have been derived from a metamorphic block which was not Dairadian. This block also contributed igneous debris which may well have come from an arc basement to the north of the Southern Uplands. BJB, JKI
Olb: Northern British Isles: mid Arenig The Ben Suardal Limestone Formation (Durness Group) is at the top of the Cambro-Ordovician sequence in Skye on the Laurentian foreland. It is at least partly of Arenig age and has yielded a rich Laurentian brachiopod fauna (Curry & Williams 1984); younger deposits presumably covered and protected this sequence. The Cambro-Ordovician shelf sea would have existed widely over the foreland now truncated on its eastern side by the Moine and related thrusts. Further north in the Durness Group, cephalopod and conodont evidence indicates that the carbonates of the Balnakiel, Croisaphuiil and much of the Durine formations are of Arenig (late Canadian/Ibexian) age (Whittington in Williams et al. 1972; Bergstrrm et al. 1985). A similar succession is present in the autochthonous sequence of western Newfoundland where shallow water carbonates of the St George Group were deposited. In the Humber Arm allochthon, shales and carbonate elastics of the Cow Head Group accumulated off the platform margin together with the basinal muds of the Curling Group. The Bay of Islands Ophiolite which, together with the other allochthonous sheets, was subsequently emplaced onto the foreland, is believed to represent ocean floor and is also of Arenig age (Dunning & Krogh 1985). The Dalradian tract of Central Scotland and northern and western Ireland underwent uplift and was intruded by basic rocks at this time The oldest uplift ages are 514 Ma and range down to c. 420 Ma, so that in Arenig times the Dalradian block had a thick cover which was being removed and was to continue to be removed until the late Ordovician to early Silurian. 19
20
ORDOVICIAN
None of the resulting sediment appears to have entered the Arenig foreland sequence of Skye, now some 75 km away from the nearest Dalradian outcrop. If Dalradian FI and F2 folds diverge to the northwest from the steep belt as they do to the southeast (see Thomas 1979), then the Dalradian outcrops would have been much nearer the foreland sequence in Skye at the time of their uplift; but since the foreland sequence is separated from the Dalradian tract by two major structures--the Moine Thrust and the Great Glen Fault--there may have been a great separation between the two areas in Arenig times. The basic masses of Aberdeenshire were intruded at about this time (489 + 17 Ma; Pankhurst 1970). Along the Highland Border, Curry et aL (1982) and Ingham et al. (1986) demonstrated the presence of an Arenig limestone, the Dounans Formation (late Canadian/Ibexian), at Aberfoyle with a diverse, silicified shelly fauna. Limestones similar to this, and thought to be of about the same age, occur along the outcrop from Arran to Stonehaven (Bluck et al. 1984). There is incompatibility in the existence of a carbonate shelf adjacent to a block which was undergoing a major phase of uplift, and the fault boundary, between the Highland Border and Dalradian rocks is regarded as a terrane boundary by Bluck (1985). Clasts of well-rounded, psammitic metamorphic rock occurring in the Highland Border sequence have yielded cooling ages of c. 1800 Ma (Dempster & Bluck 1989) and could not have come from the Dalradian block but may have come from a block in or at one time in the Midland Valley, or a block now beneath the Dalradian sequence. Probable Arenig limestone clasts which occur in a southerly derived Old Red Sandstone conglomerate (Ingham et al. 1986; Haughton 1988) near Inverbervie in the eastern Midland Valley of Scotland may relate to those at Aberfoyle and suggest widespread carbonate sedimentation at this time over the Midland Valley Terrane. In western Ireland, the rocks of the Mayo 'trough' in Co. Mayo and Co. Galway include Late Arenig sequences represented by largely volcanic and volcanoclastic rocks, with 'Pacific' graptolites, of the Glensaul Group, although brecciated carbonates (Tourmakeady Limestone) are found locally in the volcanogenic Shangort Formation yielding rich shelly faunas of Laurentian aspect (Williams & Curry 1985) closely similar to that known from the Dounans Limestone of the Highland Border Complex in Scotland (see above). Further 'north' in the Clew Bay Supercomplex, cherty and sideritic shales may extend down into the Upper Arenig (Harper et al. 1989). Immediately to the south of the subsequently relocated Dalradian terrane in Connemara lies a small tract incorporating a thick succession of pillow lavas, cherts, sandstones and conglomerates, the South Connemara Group, now believed on palaeontological evidence to be of early Ordovician, largely Arenig, age. Williams et al. (1988) preferred to correlate this group with similar rocks in the northern belt of the Southern Uplands and its Irish equivalents on a variety of structural and lithological grounds and they equated the Skird Rocks Fault, which separates the group from the Dalradian tract to the north, with the Southern Uplands Fault. However, later translation possibly of this entire crustal unit along the Doon Rock and associated faults from the main Dalradian tract substantially to the west of its present position may render such correlations less likely. It may be that the Skirds Rock Fault is a displaced segment of an extension of the Highland Boundary-Clew Bay line, in which case the South Connemara Group would have originated in a 'western' extension of the Highland Border-S Mayo back-arc basin. The problem is not resolved. The Tyrone Igneous Complex of east/central Ireland contains metamorphic and associated igneous rocks with no certain affinity to the Dalradian tract to the north. The complex also contains remnants of an obducted sheet or sheets of an ophiolite sequence which may be a northern remnant of the obducted ophiolite now seen at Girvan (Hutton et al. 1985). The Ballantrae Volcanic Complex in the Girvan area (southwest Scotland) is part of a widespread ophiolite obduction phase onto the Laurentian margin in the Arenig (Dunning & Krogh 1985). The structurally complex ophiolite comprises cherts and black shales interfingering with mass flow conglomerates overlying volcanic sequences including subaerial lavas from a variety of petrogenetic provinces and volcanogenic sediment including intertidal facies. There are also poorly preserved sheeted dykes. The age of the complex is tightly constrained and there is little difference between that of the spreading zone trondhjemitic intrusives and the cooling of the obduction-produced amphibolites. The metamorphic sole displays high P / T suggesting deep burial. These features led Bluck (1985) to favour a marginal basin setting rather than the oceanic island one of Barrett et al. (1982). Both origins, but particularly the former, suggest that the ophiolite was formerly much more extensive and this is attested to by widely occurring debris in greywackes both in the Girvan area and in the Southern Uplands. Structural studies suggest thrusting to the northwest. Graptolites of Australian and North American affinity give Early to Late Arenig (late Canadian/Ibexian) ages (Stone & Strachan 1981; Rushton et al. 1986). Late Arenig graptolites occurred in a borehole with serpentinite clasts and algal oncolites; together with the ophiolitic debris this again suggests widespread ophiolitic exposure. Palaeomagnetic studies of the ophiolites by Piper (1978) suggested postobduction overprinting whilst those of Trench et al. (1988) showed original magnetization in the Slockenray lava sequence. The latter authors thus concluded an origin within the confines of Iapetus, but involving an 80 ~ clockwise rotation. This may have occurred during ophiolite generation, or after attachment to the Laurentian margin--thus implying dextral emplacement of the block and a provenance in the northeast rather than the commonly accepted southwest (as shown).
Poorly preserved outcrops of pillow lavas, cherts and black shales crop out in the northern Southern Uplands as slices structurally interdigitating with later greywackes. If these greywackes are, as is widely believed, part of an accretionary prism, the inliers should get younger southwards as progressively younger ocean plate was incorporated into the oceanward advancing accretionary prism. This has riot been tested. BJB, JKI
O2a: Northern British Isles: late Llanvirn The Cambro-Ordovician foreland carbonate sequence of the Durness region of northwestern Scotland terminates with the Durine Formation which has yielded Laurentian mid-continent conodont faunas at least partly of Llanvirn (early Whiterockian) age (see Bergstr6m e t al. 1985). No younger units are known as the sequence is truncated to the east by the Moine Thrust belt. To what extent this sequence was once represented over the Northern Highland terrane to the east of the Moine Thrust is not known. In western Newfoundland, largely carbonate sedimentation of the Table Head Group dominates the Western Platform autochthon. A green sandstone with 'easterly'-derived ophiolitic debris above the Cow Head Group in the Humber Arm allochthon may be partly of this age (see Barnes et al. 1981) and possibly evidences a stage in the emplacement of the Bay of Islands Ophiolite (see also Cawood et al. 1988 and references therein). How this relates to developments in the northern British Isles is not known. The Central Highlands Dalradian block and its Irish equivalents were undergoing continued uplift and progressive denudation during Llanvirn times and the Aberdeen granite (470 +__ 1 Ma) and associated granites were also intruded (Kneller & Aftalion 1987; Pankhurst 1974). There is no evidence, however, of any sediment entering the foreland area, nor did any find its way to the Highland Border sequences or to any part of the Midland Valley block (Biuck 1983). Wide separation of these terranes at this time is indicated. The Dalradian rocks of Connemara in the west of Ireland also appear to have been undergoing strong uplift at this time and were thrust southwards over the rhyolitic ramp of the Delaney Dome (Mannin Thrust). It is possible that this volcanic ramp represents a displaced fragment of the Midland Valley terrane (magmatic arc). Leake et al. (1983) date these movements at 460 + 25 Ma but Tanner et al. (1989) have revised this date and put them somewhat later (447 + 4 Ma; Caradoc). It is now believed that the major sinistral displacement of the Connemara block from the main Dalradian tract along the Doon Rock and associated faults, which eventually relocated it to the south of the Lower Palaeozoic rocks of Co. Mayo, may have begun before the Mannin Thrust was initiated (see also Bluck & Leake 1986). Of the Highland Border rocks in Scotland, disrupted slivers of black shale with some sandstones and basic volcanic rocks, the supposed remnants of back-arc basin sequences, extend from Arran to Stonehaven and are believed to be largely of Llanvirn age (Bluck et al. 1984). Much of this basin has been lost subsequently by sinistral wrench faulting as well as by underthrusting of the Dalradian block by the Midland Valley terrane, movements which took place until mid-Devonian times. The Lower Palaeozoic rocks of Co. Mayo, western Ireland, contain thick sequences of clastic and volcanogenic sediments which range in age up to late Llanvirn. They are now situated to the north of the subsequently emplaced Dalradian block of Connemara and are separated from the main Dalradian terrane to the north by the Clew Bay fault zone with its disrupted ophiolite remnants and other Highland Border Complex type rocks. The subsequent sinistral emplacement of the Midland Valley block, together with details of Lower and Middle Ordovician biostratigraphy and sedimentology, are suggestive of a Midland Valley block context for these rocks. The graptolitic and shelly faunas are Laurentian in aspect (see Williams 1972; Pudsey 1984a,b; Harper et al. 1988). The youngest sediments, conglomerates of the Mweelrea and Maumtrasma formations, are partly fluviatile and indicate a progressive silting up of the Mayo ~ Whatever the nature and palaeogeographical context of the Arenig Ballantrae Volcanic Complex at Girvan, by late Llanvirn times it had become structurally integrated with the Midland Valley block by obduction and formed the foundation for a newly established 'southward'-facing proximal fore-arc basin sequence which was to persist until Llandovery times (Ingham 1978; lngham & Tripp 1991). Conodont studies (Bergstr6m 1971; Bergstr6m et al. 1985) have shown that the lower part of this sequence, exposed in the Stinchar valley to the south of Girvan (Kirkland Conglomerate to part of the Stinchar Limestone) are of this age. The Kirkland Conglomerate, of restricted and fault-controlled distribution, is the first of two mid Ordovician fan delta cycles (lnce 1984) and contains boulders, not only of Ballantrae Complex rocks, but o f granitic rocks derived from the north which have yielded radiometric ages around 481 + 28 Ma (Bluck 1983). The implication is that the axis of the Midland Valley block or terrane uplifted at about this time as it was intruded by high-level plutons which were rapidly unroofed. It became an active magmatic arc shedding debris abaxially, presumably in response to Iapetus ocean place subduction, usually interpreted as having been situated some distance to the southeast and having a northwestwards polarity. The overlying Auchensoul Bridge Mudstone, Auchensoul Limestone, Confinis Flags and Stinchar Limestone, also with limited and largely fault-controlled distribution, are essentially shallower water deposits (illaenid-cheirurid trilobite associations) whose faunas are of decided Laurentian aspect (e.g. Williams 1962; Tripp 1976, 1979; lngham & Tripp 1991).
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
22
ORDOVICIAN
Rocks of Llanvirn age in the northern belt of the Southern Uplands of Scotland and its Irish equivalents are restricted to narrow slivers ofchert and mudstone which evidently form the oceanic foundation to an overlying accretionary prism. The position of a putative subduction trench at this time is usually placed in the general position of the Southern Uplands Fault. Its proximity to the axis of the Midland Valley magmatic arc is attributed to substantial late Silurian northwesterly thrusting in the Girvan area together with probable major sinistral displacements along the Southern Uplands and associated faults (Bluck 1983; McKerrow & Soper 1989). Scandent Atlantic Province graptolites occur in the largely volcanogenic Slane Group of the Grangegeeth area of eastern Ireland (Skevington' 1974; Romano 1980; Harper & Parkes 1989). This appears to contradict the Girvan (i.e. Laurentian) aspect of the early Caradoc shelly faunas in the sequence there (see Caradoc section). Harper & Parkes recognize a 'Grangegeeth terrane' adjacent to (or within) the lapetus suture zone. Its palaeogeographical position is not clear (see p 2). JKI
O2b: Northern British Isles: mid Liandeilo No rocks of Llandeilo age are known from northern Scotland or the west of Ireland, nor are they found in the Highland Border Complex where there is evidence for a stratigraphical break at about this level (Bluck et al. 1984). Here, younger Ordovician clastic rocks rest on deformed and eroded Llanvirn, Arenig and older rocks which originated in a wide range of environments. This suggests that the Highland Border back-arc basin underwent considerable post-Llanvirn disruption; this may relate to the late Llanvirn silting up and pre-Silurian folding of the rocks in the Mayo 'trough' in western Ireland. It may also relate to the late Llanvirn uplift and onset of an intrusive history of the Midland Valley magmatic arc, but this appears largely to have been a response to the re-establishment of a putative subduction trench to the southeast. It is suggested that at least some of these Highland Border and western Ireland effects are a response to the onset of the sinistral relocation of the Connemara Dalradian block (which was completed by Silurian times) and a major phase in the sinistral accretion of the Midland Valley terrane to the Central Highlands Dalradian tract, although subduction of unknown polarity within the Highland Border backarc basin at this time cannot be ruled out. The Dalradian tract continued to rise and shed debris, but there is no evidence of any sediment reaching the Midland Valley terrane in Llandeilo times. In western Newfoundland, by Llandeilo times, the Bay of Islands Ophiolite together with the associated allochthons incorporating the basinal Curling Group and the off-platform margin Cow Head Group were in process of being emplaced or had already largely been accreted above the platformal Table Head Group and its equivalents on the autochthon. This may be evidenced by the appearance above both the Table Head Group and the allochthonous Cow Head Group of a green sandstone containing 'easterly' derived ophiolitic debris which, on the autochthon, is likely to be of this age but which on the Humber Arm allochthon may be somewhat older (see Barnes et al. 1981; see also Cawood et al. 1988 and references therein). Precisely how this accretionary event relates to the progressive terrane accretion in the northern British Isles is not clear. To the south of the axis of the Midland Valley terrane in Scotland, the proximal forearc basin successions in the Girvan area continued to accumulate in fault-controlled environments. In the main Stinchar valley outcrop to the south of Girvan, the upper part of the shallow water Stinchar Limestone, the overlying deeper water Superstes Mudstone and much of the succeeding second fan delta deposit of the thick Benan Conglomerates (Ince 1984) are all of Llandeilo (essentially late Whiterockian, including Chazyan) age (lngham & Tripp 1991). These units indicate a progressive fault-controlled foundering of the area of deposition. Towards Girvan itself, a short distance to the north, a much thinner Benan Conglomerate eventually rests directly on the Baltantrae Volcanic Complex. As with the Llanvirn Kirkland Conglomerate, the Benan contains clasts, some large, of a wide variety of rock types. Not only are Ballantrae Complex rocks in evidence, but clasts of unroofed high-level granitic plutons, some of which yield radiometric ages which are hardly different from the age ascribed to the conglomerate (559+20-467+3Ma; Bluck 1983). A variety of rocks is present, as are boulders from the underlying Stinchar Limestone which, locally, has been channelled by the Benan. One limestone boulder has yielded a small Gasconadian (early Tremadoc) shelly fauna (Rushton & Tripp 1979; see p 19). Along the south side of the Stinchar Valley, deeper water sequences are seen in the various units of the Albany group, also substantially of Llandeilo age. The Doularg Formation (Ingham & Tripp 1991) has yielded rich Laurentian assemblages with some Gondwanan elements evidencing a foundering sea floor, whilst the Craigmalloch Formation to the east has yielded graptolites with even deeper water trilobites of Gondwanan aspect, including a representative of the Cyciopygidae, from the tops of turbidite sequences (Rushton et al. MS). These faunas in particular confirm Fortey's thesis (1975) of decreasing faunal endemicity with increasing depth. The northern belt of the Southern Uplands of Scotland and its Irish equivalents consists of repetitions of greywackes above Llanvirn and Llandeilo shales and cherts, and evidences the establishment of an accretionary prism at a subduction trench at about this time (McKerrow et al. 1977; Leggett et al. 1982). Stone et al. (1987) recorded pyroxenous greywackes (Galdenoch Formation) of gracilis Biozone age that show evidence of
derivation from the south. Further south in the central belt, the Glenkiln graptolitic shales and cherts are partly of Llandeilo age and suggest that the lapetus ocean floor was the palaeogeographical setting for these deposits prior to their subsequent incorporation into the accretionary prism. Substantial, essentially sinistral relocation of the various segments of these terranes makes anything other than'a fairly generalized palaeogeographical reconstruction difficult. JKI
O3a: Northern British Isles: mid Caradoc There are no Caradoc strata preserved north of the Highland BoundaryClew Bay line and much of this area was probably land with the accreted Dalradian tract rising substantially. Cooling ages ranging throughout the Ordovician are known (Dempster 1985). The displaced Dalradian tract of Connemara was being progressively emplaced and about this time was also evidently being overthrust (Mannin Thrust) against a similarly displaced fragment of the Midland Valley terrane (the rhyolitic ramp of the Delaney Dome (Tanner et al. 1989). Largely arenite sequences preserved in the Highland Border Complex at Aberfoyle and Edzell have yielded chitinozoans which indicate a late Ordovician, perhaps mid Caradoc age (Burton et al. 1984). These rest unconformably on deformed Llanvirn, Arenig and older rocks and indicate that a restricted sea transgressed a remnant Highland Border back-arc basin at about this time. A small inlier of chitinozoabearing silts of about the same age has been described from western Ireland (Graham & Smith 1981). It is situated along a northeastward continuation of the Clew Bay line and gives some indication of the former lateral extent of this remnant back-arc basin. The Midland Valley magmatic arc terrane had evidently not finally sutured with the Dalradian tract. It was still a positive feature in Caradoc times and shed clastic sediments, occasionally conglomeratic, to the southeast at Girvan and beyond (see below). These, at the horizon represented on the map, are the upper Ardwell Group consisting, south of Girvan, largely of graptolitic and shelly shales and silts with local conglomerate tongues: they were almost certainly deposited in fault-controlled basins. By this time, however, the mid Ordovician transgression had progressed further 'northwestwards' onto the intermittently rising magmatic arc. To the northeast of Girvan, the upper Ardwell Group is represented by the Craighead Limestones, including the Sericoidea Mudstone Member with low clingani Biozone graptolites. The Craighead Limestones rest directly on an irregular Ballantrae Volcanic surface. The limestones and associated calcareous mudstones are locally highly fossiliferous (Williams 1962; Tripp 1980). Faunas are of Laurentian aspect and allow a fairly precise correlation with Shermanian sequences in the Appalachians (Ingham & Tripp, MS). Some of the carbonates at Craighead contain brecciated algal structures and were evidently deposited within the zone of wave turbulence. Other parts of the sequence represent carbonate sedimentation in inter-reef depressions and include reef talus. There are substantial developments of crinoidal limestones with rolled corals and disrupted algal masses interspersed with areas of calcareous mudstone with rich shelly assemblages and occasionally graptolites (Tripp 1980). The overlying graptolitic Plantinhead Flags indicate a continuing north or northwesterly transgression. Later Caradoc strata (the Whitehouse Group) include deep water sequences with indigenous pelagic trilobites and blind benthos (the cyclopygid biofacies) interspersed with turbidites, graptolitic shales and polymict conglomerates containing transported outer neritic assemblages. Whilst the latter retain elements of Laurentian aspect, such as Cryptolithus, Baltic and Anglo-Welsh elements make a notable appearance in such shallower water faunas. The latest Caradoc deep water cyclopygid-dionidid influx is of Gondwanan aspect and whilst not new to the fringes of Laurentia (see Llandeilo section) emphasizes the increasing similarities between the faunas around the margins of the closing Iapetus. In Northern Ireland, mid Caradoc rocks are represented by the sandy Bardahessiagh Formation and the Junction Beds (the latter of Shermanian age) in the small Pomeroy Inlier of Co. Tyrone. This shallow water sequence correlates directly with the Ardwell Group of Girvan and contains much in common with it. The base to the succession is not seen: the sequence was thrust northwards over the Tyrone Igneous Complex (? Midland Valley magmatic arc) probably in late Silurian times. The northern belt of the Southern Uplands of Scotland and its Irish equivalents contain slivers of possible accretionary prism greywackes, conglomerates and siltstones of this age yielding not only Lower Hartfeil graptolites (clingani-linearis biozones) but also locally, as at Kilbucho, shelly fossils which indicate correlation with the lower Ardwell Group at Girvan and perhaps slightly earlier horizons (A. W. Owen & E. N. K. Clarkson, pers. comm.). This material may have been transported down the trench slope or may have originated in shallow water around oceanic volcanic edifices which were moved into the subduction trench. Relevant to this possibility is the partly volcanic sequence in Peeblesshire which includes the fossiliferous Wrae limestone, a unit which appears to consist, at least in part, of large olistoliths. It may be that these accreted to the sedimentary pile in the subduction trench at the same time as the associated volcanics. Following sedimentological studies in the northern belt of the Southern Uplands of Scotland and along strike in Ireland, there is evidence of a 'lost', old volcanic arc or edifice which shed material to the north (see Stone et al. 1987; Morris 1987; Styles et al. 1989; Kelley & Bluck 1989). In the central belt of the Southern Uplands (Moffat area) and Irish equivalents, only oceanic pelagic
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
24
ORDOVICIAN
and hemipelagic graptolitic muds are known. Their present geographical position owes much to the subduction process and to substantial sinistral displacement. The Ordovician succession at Grangegeeth in northeastern Ireland, south of the Longford-Down massif (Southern Uplands equivalent), presents something of an anomaly (see Ingham & Tripp 1991, p 41). Early Caradoc neritic strata here contain much of Laurentian (i.e. Girvan) at~nities. The Grangegeeth Inlier is situated just to the south of the Colon-Navan Fault, which is usually taken as the surface expression of the Iapetus Suture, so it is possible that this sequence represents a micro-terrane substantially transported along the general line of the suture (see Harper & Parkes 1989). JKI
O3b: Northern British Isles: mid Ashgili No Ashgill strata are known north of the Highland Boundary-Clew Bay line and this area appears to have been largely erosional with continued uplift of the Dalradian tract. Any Dalradian debris transported towards the south at this time has presumably been lost along the Highland Boundary-Clew Bay suture where substantial underthrusting towards the north or northwest was taking place as late as mid Devonian times. It is possible that the younger Highland Border Group arenites are partly of this age but there is no firm evidence (Burton et al. 1984). In the Girvan area, the transgressive Ordovician sequence is dominated in the Ashgill by the Shalloch Formation and the Drummuck Group. Whilst the former (of Pusgillian-early Cautleyan age) is a relatively poorly fossiliferous flysch sequence, the Drummuck Group (late Cautleyan-Rawtheyan) consists largely of sandstones, conglomerates and mudstones, some of which show evidence of mass down-slope transport (Ingham 1978; Harper 1979, 1982a,b, 1984). The late mid Ashgill level chosen for the map is that of the upper part of the Upper Drummuck Group, which includes the level of the famous Starfish Beds. These have yielded an extensive fauna containing many phyla. Most starfish and trilobite specimens are complete and various biofacies are probably represented. Whilst it has been suggested that this unit formed as a storm deposit (Goldring & Stephenson 1972), it seems more likely that downslope transport of live faunas is responsible for the nature of the accumulation. The fauna contains many cosmopolitan elements and correlation with Anglo-Welsh and Baltic sequences at this level is relatively straightforward. Nevertheless, Laurentian elements persist, such as the trilobites Cryptolithus, Achatella and Isotelus; there are no published records of these in Britain south of the Iapetus line at this time. At Pomeroy, Co. Tyrone in Northern Ireland, the highest part of the deeper neritic Killey Bridge Formation is of this age (Ingham & Tripp, MS). The fauna has a number of species in common with the Starfish Beds of Girvan but is more limited. The northern belt of the Southern Uplands of Scotland contains little evidence of deposits of later Ashgill age. In the central belt, oceanic pelagic and hemipelagic muds, generally pale bioturbated 'Barren Mudstones', but with thin anoxic beds (D. anceps Biozone), accumulated in an area which did not receive trench clastics until late Llandovery times. Some sandstone is present at Ettrickbridgend whose origins are not clear. At Grangegeeth in northeast Ireland, south of the Longford-Down Massif, the Ashgill is possibly represented by the uppermost part of the Broomfield Formation which consists of shales with a Pleurograptus linearis Biozone graptolite fauna and by the younger Oriel Brook Mudstone Formation which has yielded a trilobite fauna similar to that from the Upper Whitehouse Group at Girvan (Williams in Williams et al. 1972; Romano 1980). Its palaeogeographical position is contentious (see Caradoc section). No Ashgill strata are known from western Ireland (Co. Mayo and Co. Galway). The emplacement of the Connemara Dalradian block and associated pre-Silurian deformation of the Ordovician rocks to the north is thought to have been largely completed by this time as early Silurian strata transgress the Doon Rock and associated faults with only minor dislocation. Similarly, it is likely that the already structurally complex Highland Border back-arc basin sequence was being substantially disrupted as the entire Midland Valley terrane was progressively docking with the still rising Dalradian tract along the Highland Boundary-Clew Bay line. JKI
O4a: Southern British Isles: Tremadoc The base of the Tremadoc is defined in North Wales, but the biostratigraphical succession is derived largely from the Shineton Shales of Shropshire (summary in Bulman & Rushton 1973). The lower Tremadoc is commonly divided into the Rhabdinopora flabelliformis [formerly Dictyonerna flabelliforrne] Biozone (below) and the Adelograptus [Clonograptus] tenellus Biozone; the former is depicted on the map. The Tremadoc is remarkable for the uniformity and thickness of its development over England and South Wales. It blankets the Midland Platform, and apparently crosses major structural lineaments with little change. A thickness of some 2 km has been estimated in the Welsh Borderland (Cave 1977, p. il; Old et al. 1987). Although no source area has been identified, derivation from Pretannia in the south is assumed. The metamorphic grade, as shown by the colour of acritarchs and by illite 'crystallinity' values determined on mud rocks (Pharaoh et al. 1987), is generally low. Soft greenish mudstones are typical almost everywhere, generally associated with bioturbated sandstone. Although local formational names have been applied
in the past, the name Shineton Shale Formation is appropriate for the lower Tremadoc rocks over the whole of this large area. Wherever the base is seen it rests conformably, or with paraconformity, on Cambrian Merioneth Series rocks. There is a striking change from the black, anoxic upper Merioneth mudstones to the greenish Lower Tremadoc mudstones which were deposited under oxygenated conditions. In North Wales, around the Harlech Dome, the Tremadoc comprises hard cleaved argillaceous rocks with sandstone units; the aggregate thickness is only about 0.5 km. Theflabelliformis Biozone is represented by the Tynllan Beds and equivalents. Tremadoc rocks are absent from Arfon and Anglesey, but it is not known whether they once crossed the Irish Sea Horst, and were removed before the late Arenig transgression. Evidence for early Tremadoc volcanicity is confined to acid tufts at the base of theflabeltiformis Biozone in the east of the Harlech Dome (Allen et al. 1981, p. 313) but the Rhobell Fawr Volcanic Group represents an important episode of late Tremadoc arc volcanism (Kokelaar 1979). It is exposed on the southeast flank of the Harlech Dome, where it unconformably overlies the Mawddach Group of Merioneth to early Tremadoc age. In the north of the Harlech Dome there are marked variations in thickness and grain size in Upper Tremadoc rocks across north-south faults such as the Ffynnon Eidda Fault (Lynas 1973, p. 495), indicating syndepositional faulting. Acritarch evidence has proved the presence of early Tremadoc argillites in the Lake District (Cooper & Molyneux 1990) and later Tremadoc turbiditic siltstones in the Lake District and the Isle of Man (Molyneux 1979; Molyneux & Rushton 1988). Coarser-grained rocks are present in the Leinster basin in southeast Ireland with derivation inferred from the Irish Sea Horst in the Duncannon area (Askingarren Formation, age doubtful-Crimes & Crossley 1968). The only proved Tremadoc beds in southern Ireland, in the Bellewstown Inlier, Co. Meath (north of Dublin), are mudstones and slide breccias that have yielded Tremadoc acritarchs (Brenchley et al. 1977; MacAoidh in Smith 1981). It is probable that the basin of Tremadoc deposition extended from the eastern Midlands to the Brabant area of Belgium, where cleaved argillites of flabelliforrnis Biozone age resemble the Welsh Tremadoc (Vanguestaine 1977). Although there was a very widespread and presumably eustatic sealevel rise at the beginning of the Tremadoc Epoch, this alone cannot account for the great thickness of the Tremadoc Series over much of Britain. It is suggested that foundering of the Midlands microcraton at this time may have created an extensional basin over the area, and that subsequent inversion may account for the wide subcrop of gently folded but unmetamorphosed Tremadoc rocks. AWAR, RAF
O4b: Southern British Isles: Arenig The Arenig Series takes its name from Arenig (sometimes Arennig) Fawr in north Wales. Although a major division of the Ordovician System, the Arenig has not been studied in detail until recently. A revision of the bioand lithostratigraphy has shown that the development of the series is very incomplete in the type area, where the middle part and most of the upper part are missing. The most complete succession is in south Wales (Fortey & Owens 1987) where basinal sediments resembling those of the Tremadoc Series accumulated after a regression in latest Tremadoc times. Elsewhere, sedimentation is apparently fault controlled, and local hiatuses are common. The Series has been divided into three stages: Moridunian, Whitlandian and Fennian, in ascending order (Fortey & Owens 1987). The map shows the geography of late Moridunian times. The basal Arenig is transgressive, following the late Tremadoc regression; this is shown by a change in biofacies, with the local base characterized by the shallow water Neseuretus trilobite biofacies in arenaceous lithologies, even where there is no evidence for angular conformity. In the western Ll~,n the lower Arenig transgresses Precambrian rocks (Beckly 1988). In central south Wales some 1300 m of Arenig strata are present, mostly mudstones and turbidites which may have been derived from both southerly and northerly sources during the initiation of a marginal basin (Traynor 1988). The basal Arenig rocks are of shallow water arenaceous facies, containing the trilobite Neseuretus, various articulate brachiopods, and bivalves. The Carmarthen Formation of the Carmarthen area (later Moridunian in age) is developed in a distinctive euxinic facies (Fortey & Owens 1978), which was probably caused by a barrier in the Haverfordwest region that restricted oceanic circulation. There are also local differences in lithology and stratigraphy in the mid Arenig (Whitlandian) which may partly reflect basement control on sedimentation. By Fennian times, however, a distinctive open ocean biofacies with graptolites and pelagic trilobites had spread across south Wales. In the St David's area of western Wales the succession is generally thinner. In north Wales the palaeogeography is more complex, and recent studies by Beckly (1987, 1988) have revealed very different distributions of facies and faunas for the Moridunian, Whitlandian and Fennian (Fig. 1). Fault movements controlled the scale and timing of sedimentation, with marked contrasts between the western Ll~n, Anglesey, and the Harlech Dome; only in western LIS'n is there a relatively complete succession. The lower Arenig of the type area and the Migneint is represented by the Henllan Formation (inshore Neseuretus biofacies) and the underlying Llyfnant Flags (Zalasiewicz 1984; Lynas 1973). The Moridunian, developed in a mudstone facies in the western Ll~n is not recognized on Anglesey.
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
26
ORDOVICIAN
Fig. 1. Facies distribution in North Wales during the Arenig (after Beckly (1987) Geological Journal, 22, 26). Legend: gridded = upstanding area; stippled = shallow water Neseuretus and flaggy sandstone facies; dashes and dots = siltstone facies; dashes only = isograptid biofacies siltstones; mixed ornament = mass-flow deposits; v = volcanics. Reproduced by permission of John Wiley & Sons Ltd. In the Shelve district of Shropshire the base of the Arenig is locally transgressive and is taken at the base of the Stiperstones Quartzite, a crossbedded orthoquartzite with a few body fossils but many Skolithos burrows. The succeeding Mytton Flags Formation may represent much of the Arenig but is entirely in relatively shallow water facies as compared with south Wales. In the Lake District the Skiddaw Group is a predominantly argillaceous and turbiditic sequence, sparsely graptolitic throughout (Jackson 1978). With the recognition of strata below the Didymograptus deflexus Biozone (Rushton 1985; Molyneux & Rushton 1988), the sequence probably includes the whole of the Arenig Series. Trilobites from the Skiddaw Group, though rare, are of typical marginal Gondwana type (Fortey et al. 1989). Comparable sequences generally lacking macrofossils are recognized in the Manx Slates of the Isle of Man (Molyneux 1979). In the Lake District northwestward slumping at about the end of the Arenig Epoch emplaced a major olistostrome (Cooper & Molyneux 1990). The turbiditic siltstones of the Ingleton Group (west Yorkshire) may possibly be of early Arenig age (Arthurton et al. 1988); slump folds suggest a northwesterly-facing palaeoslope there. In southeast Ireland the shallow-water shelly and graptolitic sandstones and shales at Tagoat, south of Rosslare, are post-Moridunian in age and rest directly on Precambrian rocks (Brenchley et al. 1967); they indicate continued Arenig transgression after the interval depicted on the map. Further north, near Courtown, the alternating siltstones and mudstones of the Ribband Group indicate a more distal facies in beds which may be of similar age. Since the map was drafted, Max et al. (1990) have recognized several terranes in this area; they are not discriminated on the map. There was little volcanic activity during Arenig times. Some tuff horizons have been recognized in the Llgn Peninsula and on Ramsey Island in southwest Wales. The volcanic rocks reported in the Manx slates (Molyneux 1979, p. 419) are thought to be of early Arenig age (S. G. Molyneux, pers. comm.). The arc-like Trefgarne volcanic rocks of the Haverfordwest district in South Wales are often regarded as Tremadoc, by comparison with the Rhobell Fawr Volcanic Group, but an early mid Arenig age cannot be discounted from the known field relationships. RAF, AWAR
O5a: Southern British Isles: late Llanvirn The Llanvirn Series has its type development in south west Wales, where it is divided into two Biozones: D. artus (formerly known as D. bifidus) and D. murchisoni in ascending order. The map represents palaeogeography of the upper of these two divisions. The Llanvirn represents a time of transgression, after a latest Arenig regression, producing an extensive blanket of generally argillaceous and turbiditic rocks over much of Wales, and extending to the margins of the
Midland Platform. Unlike the Arenig, voicanics are abundant almost everywhere. Following the transgression the inshore Neseuretus trilobite biofacies retreated from the Anglo-Welsh area on to western Gondwana (Armorica, lberia) in the Llanvirn (Fortey 1984). As a consequence, deep Water biofacies dominated in the Welsh Basin and in Shropshire. In the Hope Shales (artus Biozone), Shelve Inlier, for example, dark mudstones yield a distinctive trilobite fauna with giant-eyed cyclopygids as one of the commonest elements (Whittard 1979). Cyclopygids were probably mesopelagic, and spent much of their time in the water column. They are associated with other, benthic trilobites which, by contrast, are often blind, or nearly so, even though many of them have close relatives with 'normal' eyes. Such atheloptic (Fortey & Owens 1987) trilobite assemblages may have lived below the photic zone. Many Llanvirn outcrops are very poor in fossils, and it is likely that the basin was subject to periodic anoxia. The characteristic pendent didymograptids are generally present, however. Their appearance at the base of the Llanvirn is abrupt, although other faOnal elements show intergradation with those of the late Arenig Epoch. Along the margins of the Midland Platform rocks of the murchisoni Biozone are represented in the Builth and Shelve areas. In the Shelve area the Weston and Betton Beds unconformably overlie the Stapeley Volcanic Formation (Lynas 1983); at Builth apparently deeper water shales lie above and below the Builth Volcanic Group. This group comprises a subaqueous sequence of basalts, dacites and rhyolites, with associated high-level intrusions and various volcaniclastic and pyroclastic rocks. All have calc-alkaline affinities (Kokelaar et al. 1984). Across western Wales the faunas of the murchisoni Beds as a whole suggest deep water biofacies, with shallower equivalents in the Ffairfach Group (Lockley 1983), but a non-sequence in the Haverfordwest area suggests continuation of a positive area there. Volcanic activity continued in the Fishguard area with eruption of bimodal, tholeiitic basalts and rhyolites and associated high-level intrusions (Bevins 1982). Rhyolitic pyroclastic products become more important to the east in the Newport area (Lowman & Bloxam 1981). Another major volcanic centre was on Ramsey Island to the west (Kokelaar et al. 1985). In North Wales the Maesgwm Slates and correlatives are monotonous argillites extending from Cader ldris to Anglesey. Varied voicanics are present locally, particularly flanking the Harlech Dome region, in the Cader ldris, Aran Mountains, Manod and Moelwyn areas, dominated by bimodal tholeiitic basalts and rhyolites and related rocks (Kokelaar et al. 1984). In the Lake District there are dark mudstones, those of the artus Biozone including tuff interbeds (Wadge 1978). The Eycott Volcanic Group, formerly thought to be of early Llanvirn age, is now considered to be later, possibly of early Caradoc age (D. Millward & S. G. Molyneux MS). In Ireland the succession at Grangegeeth and Bellewstown includes basic tufts with an associated trilobite and graptolite fauna (summarized in Brenchley et al. 1977). The conjectural east England Caledonian belt is based on borehole evidence including faunas possibly of this age (Rushton & Hughes 1981; Jenkins 1983) extending as far as Belgium (Vanguestaine 1977). Altered felsic lavas in the Cox's Walk Borehole provide a radiometric date of 466 + 11 Ma. All are presumed to lie at the edge of the Tornquist Sea and are thought to be part of an early Palaeozoic arc-related volcanic complex extending from the Lake District to Brabant (Pharaoh et al. 1991). In a wider context, the Llanvirn essentially continues the main features of Arenig palaeogeography, although there is an onstep of generally deeper water facies southwards over western Gondwana (Cocks & Fortey 1988). This is reflected in many sections of richly fossiliferous rocks in Iberia and Armorica overlying the relatively shallow water quartzites of Gr6s Armoricain type. Southern British volcanic rocks reveal a complex association, with varying degrees of subduction zone influence (Kokelaar 1988). In Wales the volcanics have been described as having been erupted and emplaced in a marginal basin environment, developed on the flanks of a continental margin dominated by crustal tension. Available evidence is equivocal as to the polarity of the associated subduction zone, often considered to be reflected in the arc volcanic sequences of the Lake District and Leinster. RAF, REB
O5b: Southern British Isles: mid Llandeilo The Llandeilo Series is considered to lie within the graptolite biozones of Glyptograptus teretiusculus and Nemagraptus gracilis (Whittington et al. 1984; Bergstr6m et al. 1987). Despite recent work on the graptolite sequences of the Welsh Borderland (Hughes 1989) there are still difficulties of correlation in the teretiusculus Biozone, and in correlating between the graptolitic and the shelly facies such as is developed at Llandeilo itself. Furthermore, Bergstr6m et al. (1987) have indicated that much of the G. teretiusculus Biozone is pre-Llandeilo (and thus Llanvirn, by definition), and since the later part of the N. gracilis Biozone is post-Liandeilo in age, positive identification of Llandeilo strata within poorly fossiliferous graptolitic sequences can be very difficult. The overall palaeogeographical picture is similar to that of the Llanvirn, with a blanket of marine sediments over much of Wales and extending onto the western margins of the Midland Platform. The position of the northern edge of the Midland Platform is uncertain and no direct evidence is available regarding the position of the eastern edge. Volcanism is generally lacking in
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
28
ORDOVICIAN
the southern part of the region but is believed to become more widespread northwards (but see below). The sequences of South Wales and the Welsh Borderland suggest relatively shallow marine areas deepening towards the centre of the Welsh Basin, the original width of which is not known but is likely to have been significantly wider than shown. A variety of medium- and fine-grained clastic facies are developed which are locally calcareous, for example at Llandeilo, and in places unconformable on the underlying Llanvirn strata. Outcrops other than in the type region and in the Shelve area of Shropshire generally yield a graptolitic fauna though there are shelly faunas known from the Berwyn Hills, north and west of which the sequences are generally poorly fossiliferous. This causes serious difficulties in the recognition of Llandeilo sequences in the northern part of the region. Though some of the volcaniclastic sequences in North Wales, particularly those in the ArenigBala and Corris-Cader Idris regions, have been considered to be of Llandeilo age, there is little evidence to prove that this is the case, for although Glyptograptus teretiusculus has been recorded from a number of localities no assemblages are known that allow confident recognition of the zone. Restudy of faunas from Tyddym-dicwm (Tremadoc area) has indicated that a gracilis Biozone fauna is definitely present (A. W. A. Rushton, pers. comm.). In much of northwest Wales there appears to be a break in sequences following the Llanvirn, though whether or not this indicates emergence or areas of subsequent erosion is not clear. The lower parts of these sequences generally remain equivocal in age, and though recent work (e.g. Campbell et al. 1988, Howells et al. 1983) has confirmed that much of the sequence is of Caradoc age, it is thought probable that some at least is of Llandeilo age (Kokelaar 1988) and is indicated as such on the map. In the Lake District the Borrowdale Volcanic Group has long been considered to be of Llandeilo age although palaeontological evidence for this has been lacking. It overlies sediments of the Skiddaw Group of late Llanvirn age and is overlain in places by rocks of mid Caradoc age (Branney & Soper 1988). Radiometric dates are also inconclusive and as yet not really precise enough, with a Sm-Nd garnet age of 457+9 Ma being obtained by Thirlwall & Fitton (1983) which spans the Llandeilo--early Caradoc on the timescale of McKerrow et al. (1985) and the scale adopted herein. Molyneux (1988) has suggested that, on the basis of new micropalaeontological evidence, a Caradoc age is possible, if not probable, for at least a part of the sequence, although a Llandeilo age cannot be ruled out. Thus although recent work (e.g. Firman & Lee 1986) has greatly increased our understanding of the Lake District Ordovician sequence we must await further work before being certain of the timing of many events and hence of the palaeogeography of that region. The present map makes the assumption that some volcanic activity was present in the region in Llandeilo times, and that it continued into the early part of the Caradoc. In much of southeastern Ireland there is a substantial break in the sequence in post-Llanvirn times (Holland 1981) and the precise palaeogeography of the area remains uncertain. The calcareous shales and limestones of the Bellewstown beds and the Tramore Limestone may represent the earliest beds of a Llandeilo transgressive phase in the region that continued into Caradoc times (Carlisle 1979). In the southwestern extremity of England some faunas of undifferentiated Llandeilo age occur (Bassett 1981; Sadler 1974) in what are thought possibly to be major olistoliths within the late Devonian Roseland Breccia Formation. These faunas occur within quartzites of Armorican type and are difficult to correlate with other faunas from the British Isles. Indeed, they may only be correlated approximately by the use of slender graptolite evidence from the Armorican sequences (Henry 1980); it is thus impossible to determine how these occurrences relate to the remainder of the palaeogeography of the southern part of the British Isles. CPH, JKI
CARADOC The Caradoc has its standard succession in south Shropshire where it is divided into seven stages on the basis of the rich brachiopod and trilobite faunas: Onnian Actonian Marshbrookian Longvillian Soudleyan Harnagian Costonian Some of the stages are divided into biozones. Approximate correlations can be made between these zones and stages based on shelly faunas and the graptolite zonal stratigraphy which has been established in parts of the Welsh Basin. Palaeogeographic maps are given for four of the stages (Costonian, Harnagian, Longvillian and Onnian). Hurst (1979b) introduced a Woolstonian Stage for the upper part of the Longvillian. In this account the use of Longvillian includes the Woolstonian of Hurst. The broad outlines of Caradoc palaeogeography were initially determined by movements on the Church Stretton/Pontesford-Linley lineaments which separated the Midland platform from the Welsh Basin, and by the disposition of the various volcanic centres in Wales, the Lake District and eastern
Ireland. The Irish Sea Horst complex which formed the northwestern margin of the Welsh Basin was subdued in relief or was submerged throughout most of the Caradoc. Widespread volcanicity persisted until mid-Caradoc (Longvillian) times in Wales and possibly until the end of the Caradoc in Ireland. Volcanicity in the Lake District was probably partly pre-Caradoc and is unlikely to have persisted long after the beginning of the epoch. Although most of the volcanic activity is associated with basin areas there are volcanic rocks on the Midland Platform, e.g. in the North Creake Borehole (Norfolk), in boreholes in Northamptonshire and at outcrop in Leicestershire, which could be of Caradoc age (Pharaoh et al. 1987). Further volcanics, probably of Caradoc age, occur at outcrop and in boreholes at Parys Mountain in Anglesey which is part of the Irish Sea Horst Complex. It is thought that the Caradoc volcanicity was related to a southeasterly directed subducting plate. The volcanism of the Lake District and eastern Ireland is of calc-alkaline volcanic-arc type, whilst that of Wales is of a mixed bimodal-type and is apparently related to crustal extension in an ensialic marginal basin (Kokelaar et al. 1984). Throughout most of the Caradoc the northern part of the Welsh Basin was characterized by relatively shallow neritic sediments while the southern part had deep neritic or basinal muds. During transgressions the basin was prone to widespread anoxia and these conditions tended to persist longer in the southern part of the basin. In eastern Ireland and the Lake District the Caradoc palaeogeography was strongly influenced by the presence of volcanic centres or residual volcanic terrains of different ages and variable size. Away from the volcanic islands neritic sediments were widespread, but anoxic black muds were again developed during transgressive episodes. The major lineaments which define the basin and horst structure of the region had a major vertical component to their movement, but may also have had large-scale strike-slip displacements. It is likely that there were strike-slip movements on the Church Stretton/Pontesford-Linley lineaments during the Ordovician so that the geographical relationship between the Midland Platform and the Welsh Basin is uncertain. Ordovician strike-slip movements in Anglesey and along the northern margin of the Irish Sea Horst Complex may well have substantially modified the geography of the region and displaced rocks of the Leinster-Lake District Basin relative to those of the Irish Sea Horst Complex (cf. Max et al. 1990). If the Lake District and Irish volcanics belonged to a volcanic arc, a trench would be expected somewhere to the north. However, no evidence of such a trench has been discovered either in the north of England or Ireland where instead neritic sediments are found close to the line of the postulated suture. It is possible that the trench, if it existed, has been subducted beneath the Southern Uplands, or moved laterally by strike-slip movements during later continental collision.
O6a: Southern British Isles: Caradoc (Costonian) In the type area of the Caradoc in south Shropshire the lowest Caradoc stage, the Costonian, lies above an unconformity and oversteps from Shineton Shales (Tremadoc) on to Precambrian rocks. The Costonian sequence starts with conglomerates and coarse sandstones (the Hoar Edge Grit) but fines upwards into laterally variable flaggy and bioturbated sandstones. A similarly transgressive sequence is known locally in Anglesey at the opposite margin of the Welsh Basin, but at other places on Anglesey lower Caradoc siltstones and mudstones lie with paraconformity on Arenig rocks. In basinal environments the Costonian transgression promoted the development of black graptolitic shales. The Midland Platform has been shown on the palaeogeographic map as a land area except for its western edge where the transgression was progressively submerging the basement of Precambrian and older Palaeozoic rocks. In the Welsh Basin, around the Harlech Dome, there had been a prolonged phase of volcanicity starting in the Arenig and possibly persisting into the early Costonian. The volcanicity, represented by the Aran Volcanic Group, appears to have been related to north-south fractures, the Rhobell Fawr lineament, on the east side of the Harlech Dome. Most of the volcanic region was submerged during the early Caradoc transgression. In the Lake District the Borrowdale Volcanic Group (and also following D. Millward & S. G. Molyneux MS, the Eycott Volcanic Group) succeeds Skiddaw Group rocks with Lianvirn graptolites and are covered by Caradoc rocks belonging to the Longvillian Stage. By Longvillian times the volcanic pile had been tilted and deeply dissected so that volcanicity must have ceased somewhat earlier. It is not possible to know whether there were active volcanoes in the early Caradoc but there must at least have been a substantial volcanic terrain above sea level. There was also early Caradoc volcanicity in eastern Ireland near Grangegeeth and in southeastern Ireland at Tramore. In areas, both in Wales and Ireland, removed from the influence of volcanic islands, and on topographic highs (such as the Irish Sea Horst Complex) there was widespread development of black graptolitic shales. In places around the margin of the Welsh Basin (e.g. Carmarthen, Arenig Fawr) impure limestones with a rich shelly fauna developed in neritic environments. Similar impure limestones occur at Tramore, Courtown and Bellewstown in eastern Ireland and are succeeded by black shales with a Nemagraptus gracilis fauna (Brenchley et al. 1977). The distribution of faunal associations at an early stage in the Costonian transgression is shown in Fig. 2.
29
30
ORDOVICIAN
Table 1. Characteristics of the various benthic assemblages in the Caradoc of the Southern British Isles area Benthic assemblage
Environment and estimated depth
Character of fauna
Associations
1
Shoreface 0-15 m
Abundant, low diversity brachiopod faunas
Dinorthis (D)
2
Inner neritic (inner shelf) 15--40m Mid-neritic (mid-shelf) 40-70 m Outer neritic (outer shelf) 70-120 m Shelf/slope edge
Abundant, moderate diversity brachiopod faunas + trilobites High diversity brachiopod and trilobite faunas
Bancroftina (B), Oalmanella (D_), Howellites/ Kloucekia ( H_), Heterorthis ( H ) Nicolella (N) Onniella (O), Onniella/Sericoidea (Q), (Graptolites (G))
Novaspis/cyclopygid (~1), Graptolites (G)
3
6
Slope
Sparse, high diversity, brachiopod and trilobite faunas Sparse, high diversity trilobite fauna and low diversity branchiopod fauna Sparse, moderate diversity trilobite fauna
7
Deep slope/basin
Sparse, low diversity
4 5
Onnia (0), lllaenid/Cheirurid (C), (Graptolites (G)) Nankinolithus/Opsimasaphus (_N), Graptolites (G)
The numbers on the accompanying maps indicate the benthic assemblage, and letters refers to particular faunal associations (see,above). Where the faunas are only generally known they have been assigned to a benthic assemblage i.e. 1 to 7. Graptolite faunas occurred in environments ranging from outer neritic to deep basin, depending on the circulation within the basin. Blocks of bioclastic limestone are found in early Caradoc debris flows in northern Anglesey implying the presence of a shallow marine area bounded by steep marine slopes nearby. The blocks are believed to have come from a source to the north.
O6b: Southern British Isles: Caradoc (Harnagian) The lower Caradoc transgression continued into Harnagian times so that marine areas were enlarged and deepened. In south Shropshire, east of the Church Stretton lineament, Harnagian deposits with a thin basal conglomerate overlap on to Precambrian rocks. On the Midland Platform the Harnagian is mainly represented by mudstones, the Harnage Shales, with a varied shelly fauna. In west Shropshire and North Wales the Harnagian sediments are generally deep neritic mudstones or black shales. Black anoxic shales prevail in the mid- and south parts of the Basin. In the Lake District and eastern Ireland volcanicity may have persisted throughout Harnagian times, but the dating of the rocks is uncertain.
O7a: Southern British Isles: Caradoc (Longvillian including Woolstonian) The principal change in palaeogeography which occurred between the Costonian and Soudleyan arose from a shift in the location of volcanic centres in North Wales. The volcanicity around the Harlech Dome ceased in the Costonian and new centres developed in northern Snowdonia, the Berwyn and Breidden Hills and the Shelve Inlier (Fig. 3). The volcanicity in and around Snowdonia appears to have been related to two major grabens, the Pitts Head Graben and the Snowdon Graben which had an approximately NNE-SSW trend (Fig. 4) (Kokelaar 1988). The earliest of the volcanics occurred in northern Snowdonia and is represented by the Llewellyn Volcanic Group (Soudleyan) which is dominated by voluminous ash flows. Their eruption followed uplift along the Menai Straits lineament and the development of a narrow northwest-southeast horst in the Conwy Fault System. This produced a fault block, tilted to the south or southeast which was subaerial in its northern part but had coastal and then deeper marine environments towards the south. The Cwm Pennant lineament was also active in the Soudleyan and controlled the eruption the Llwyd Mawr ignimbrite consisting of over 700 m of welded tuff, mainly contained within a narrow graben or caldera (Kokelaar 1988). There were two outflows to the east of the lineament. The older of these is the Pitts Head T u f t Formation which extends for 20 km to the northeast and passes progressively over alluvial fans, alluvial plain sediments, shoreface and marine shelf environments. It terminates rather abruptly against the east side of the northeast-trending graben. The second outflow is an ash flow tuff, restricted in its northeasterly extent and locally deformed by sliding and slumping, suggesting contemporaneous tectonism. Fault controlled volcanism is also seen along the northeast-trending Yr Arddu fracture which controlled local uplift and associated fissure eruptions
Costonian
Fig. 3. Distribution of Caradoc volcanic centres in North Wales. The position of the centres of the 'other volcanics' is estimated. (Howells et al. 1987). Similar fault controlled volcanism occurred in a narrow zone along the Moel Ddu-Cwm Llan Fault Zone where pillowed basalts and rhyolitic tufts were erupted. Subsequently, about the end of the Soudleyan, a major resurgent caldera developed in the Snowdon region (the Snowdon Caldera) in which up to 500 m of rhyolitic ash flow tufts (Lower Rhyolitic Tuff) accumulated locally in Lower Longviilian times. There were outflow facies extending east and northeast into the northeast-trending and deepening basin. Following the rhyolitic eruptions there was widespread basaltic volcanism in central Snowdonia (the Bedded Pyroclastic Series) and locally hyaloclastites and pillow lavas formed the base for small volcanic islands. About the same time in northern Snowdonia a major volcanic centre developed along splays of the Conwy Valley Fault and formed a basaltic pile more than 500 m thick (Tal y Fan Volcanic Formation, Howells et al. 1985). Further volcanicity in this region is seen in the thick development of submarine rhyolitic tufts (Crafnant Volcanic Formation, Howells et al. 1983) and the thick basaltic pile of the Dolgarrog Formation (Howells et al. 1981). The siting and facies-distribution of both these volcanic developments appears to have been influenced by movements within the Conwy Fault Complex. Elsewhere in Wales there was a Soudleyan volcanic centre to the northwest of the Berwyns producing widespread ash flow tufts (the Swch Gorge Tuft) around the north and west flanks of the Berwyn Hills and a volcanic centre nearby to the east producing a mainly subaerial sequence of pumice rich ash-fall deposits (Cwm Ciwyd Tuff). Emergent andesitic volcanoes were present in the Breidden Hills and at Montgomery in east north Wales. These had a thick accumulation of cobble and boulder conglomerates deposited by mass flows at the submarine base of the volcanic cone. The more distal representations of this ~'olcanism are seen in the Shelve Inlier as the Hagley and Whittery Tufts.
Onnian
Fig. 2. Distribution of benthic assemblages in the Costonian, Longvillian and Onnian stages. For key to symbols, see Table 1.
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
32
ORDOVICIAN
ASHGILL The type area for the Ashgill Series is taken in the Cautley inliers in northern England (lngham et al. 1978). The Ashgill is divided into four stages on the shelly faunal succession there: Pusgillian, Cautleyan, Rawtheyan, Hirnantian. Graptolites are rare, so correlation of the stages with the graptolite zones, which are based on the faunas of the Moffat Shales ofsouthern Scotland, are indirect and somewhat uncertain. The base of the Ashgill Series is thought to lie at a level within the P. linearis Biozone. Fig. 4. Structural control of the Snowdon Volcanic Group. PHG, Pitts Head Graben; SG, Snowdon Graben (after Kokelaar 1988). In the Longvillian a volcanic centre to the northeast of the Berwyns formed the ash flow tufts of the Pandy Tuff Formation. In the Ll~n Peninsula there was a rhyolite-dominated belt of domes, composite volcanoes and pyroclastic sheets (Leat & Thorpe 1986). The widespread volcanicity in north Wales provided abundant volcanic detritus which was reworked in generally shallow neritic environments (Brenchley & Pickerill 1980), often showing the effects of storm-influenced sedimentation. Rather deeper water muddy environments were present in the Shelve and Breidden areas. In the central and southern part of the Welsh Basin black graptolitic shales were deposited throughout most of the Soudleyan and Longvillian. The distribution of benthic assemblages reflects the broad distribution of environments (Fig. 2). Within the basin sequence there is one major influx of coarse clastics, to be seen at Poppit Sands on the Dyfed Coast near Cardigan, which represent deep-sea fan deposits, probably of mid Caradoc age (James 1975). In south Shropshire at the western margin of the Midland Platform shallow-marine sandstones, commonly glauconitic, were deposited during the Soudleyan and Longvillian. Locally there are nearshore pebbly sandstones containing fragments of Uriconian in the Chatwall Sandstones (Soudleyan), suggesting localized uplift along the Church Stretton Lineament. There is widespread evidence of storm sedimentation in the Horderley Sandstone and in the overlying Alternata Limestone (Woolstonian [= Longvillian] Stage; Hurst 1979b) where members of different formal associations are found mixed in storm-deposited coquinas (Hurst 1979a) In the northern part of the Lake District and at Cross Fell Longvillian rocks succeed the Borrowdale Volcanics with unconformity and mark the initiation of a transgression from the north across the tilted and eroded volcanic strata. In eastern Ireland there was active acid to intermediate volcanicity in the belt from Courtown to Tramore with mainly submarine volcanic centres. Further north at Balbriggan, Herbertstown and Grangegeeth there was midCaradoc volcanicity, but the stratigraphy is different in adjacent fault blocks. It has been suggested (Murphy 1987) that the blocks represent dismembered parts of a volcanic arc system juxtaposed by strike slip faults. Between the volcanoes there were mainly neritic sediments with a varied brachiopod-trilobite fauna.
O8a: Southern British Isles: mid Ashgill (Rawtheyan) In early Ashgill times there was apparently a general shallowing of the Welsh Basin accompanied by a change from the generally anoxic black graptolitic shales of the Onnian and Pusgillian to the generally monotonous grey shales of the later Ashgill Series. A similar colour change is found in the shale sequence at Moffat, which would have been on the other side of the Iapetus Ocean at this time (see O3b), so it is possible that the sea-level change at these widely separated localities was eustatic rather than a purely local one. In North Wales there is a widespread break between the Caradoc and overlying Ashgill with possibly Pusgillian and Cautleyan rocks generally missing. At Bala, Caradoc rocks are folded and overlain unconformably by strata of Rawtheyan age but elsewhere Rawtheyan strata lie disconformably over the Nod Gias. The stratigraphical break appears to be generally the result of sub-marine erosion and non-deposition since deep neritic rocks overlie anoxic shales with no intervening shallow marine deposits. A deep neritic environment with mixed Rawtheyan brachiopod and trilobite assemblages prevailed across North Wales, and along the southeast edge of the basin from Haverfordwest to Llandeilo. The band of outcrop which runs from Bala southwest to Towyn shows sediments which apparently occupied a position progressively further down the palaeoslope in a southwest direction (Price 1980). Thus Rawtheyan mudstones at Bala have a very diverse, deep neritic trilobite fauna whilst a deep-water Novaspis/cyclopygid association is found near Corris (Fig. 5). Anoxic graptolitic muds were apparently confined to the deepest part of the basin, represented by rocks on the Dyfed coast. In the north of England, on the southern flanks of the Lake District, the last remnants of the land area formed of Borrowdale Volcanics and earlier rocks was submerged during the Cautleyan. However, there are two substantial disconformities within the Ashgill succession which suggest that the area remained relatively positive throughout Cautleyan and Rawtheyan times. Elsewhere in the north of England mudstones with calcareous nodules contain a mixed trilobite-brachiopod fauna, suggesting a fairly deep neritic environment (Ingham et al. 1978). A large carbonate mud mound developed at Keisley, a carbonate mound of similar nature is known from Kildare in eastern Ireland, and bedded micritic limestones with a diverse fauna are found at Portrane, suggesting that locally there were slightly shallower environments. However, exposure of Ashgill rocks in Ireland are rare and the palaeogeography is consequently obscure.
O8b: Southern British Isles: late Ashgili (Hirnantian) O7b: Southern British Isles: Caradoc (Onnian) A regional transgression starting in the Marshbrookian produced a progressively deeper water sequence in the neritic deposits of the Welsh Borderland and a condensed succession of black shales elsewhere. In Shropshire there is a change in the nature of the benthic communities, from the relatively shallow neritic Dalmanella unguis association in the Marshbrookian, to the deep neritic Onniella broeggeri association in the Onnian (Fig. 2). In the Welsh Basin the transgression is reflected by the presence of a nonsequence or by black anoxic graptolitic shales. Black shale deposition was probably established at a relatively early stage in Snowdonia, where shales with a Diplograptus multidens fauna are found. The last burst of volcanicity, represented by the Trefriw Tuff, is also recorded in this area. Elsewhere in North Wales there was a widespread disconformity where Longvillian sediments are covered by thin phosphatic deposits of Onnian age and then by the black shales of the Nod Glas with a linearis Biozone fauna. By late Onnian times most of Wales was blanketed in anoxic black muds. In the north of England the land-area of folded and uplifted Borrowdale Volcanics and older rocks, possibly in the form of a horst block, was being progressively covered by the transgressive sea but was not finally inundated until the Ashgill. To the east in the Cross Fell inlier, the Dufton Shales, ranging from Longviilian to Onnian age and extending into the Ashgill, contain a trilobite-dominated fauna reflecting deposition in muddy deep shelf environments. In Ireland volcanicity continued until the late Caradoc at Portrane, and possibly at Kiidare and in Southeast Ireland. On the Waterford coast the volcanics are succeeded by the Raheen Shales, which have deep shelf trilobite faunas and are of late Caradoc age (Briick et al. 1979). Away from the volcanic centres, as at Grangegeeth, the late Caradoc transgression led to the development of black graptolitic shales whose fauna indicates the presence of the D. clingani Biozone and possibly the P. linearis Biozone. PJB
A glacio-eustatic drop in sea level estimated to be of the order of 50-100 m during the Hirnantian radically changed the palaeogeography of the British Isles. Sea level probably started to fall at the beginning of the stage when the generally low-diversity inner-neritic Hirnantia fauna appeared in those areas previously occupied during the Rawtheyan by relatively higher-diversity deep-neritic faunas. A Hirnantia fauna is also found capping the deep-neritic carbonate mud mounds at Keisley in the north of England and Kildare in Ireland. Areas in the north of England where calcareous, nodular mudstones had accumulated earlier in the Ashgill were blanketed by the grey, noncalcareous Ashgill Shales, possiblyreflecting changes in water temperature and circulation early in Hirnantian times.
Fig. 5. Distribution of benthic assemblages in the Rawtheyan stage. For key to symbols, see Table of benthic assemblages and faunal associations (Fig. 2).
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
34
ORDOVICIAN
The main drop in sea level occurred slightly later in the Hirnantian (Brenchley & Newall 1984). In neritic areas the seas became shallow or the areas were temporarily exposed. In North Wales well sorted sandstones, frequently containing a low-diversity Hirnantia fauna, are found filling broad channels cut into mudstones, as at Meifod and Corwen. In the north of England channels filled with coarse angular clasts are recorded from Cautley and massive limestone breccias filling channels are seen near Broughton-in-Furness in the southern Lake District. A similar breccia-filled channel is seen at Portrane in Ireland. Deep erosion occurred near the shelf edge, as at Bala, where more than 100m of Upper Ashgill sediments are missing. Coarse sediments were carried in channels across the shelves and into basin areas at this time, to form a variety of gravity-flow deposits. Mass-flow deposits are seen in the Drosgol Formation at Plynlimon, deep sea fan complexes at Llangranog on the Dyfed coast, and channellized pebbly sandstones, massive sandstones and turbidites with Bouma sequences parts of submarine fans near Machynlleth and Corris (James 1985, 1987). Slumps generally indicate a northwest-facing palaeoslope to the eastern side of the Welsh Basin. Of particular interest are a sequence of massive and graded turbidites near Conway which have an important component of bioclastic carbonate and ooids, and which appear to have been derived from a shallow carbonate platform to the north, in the region of the Irish Sea Horst Complex. The low-stand of sea level lasted until the end of the Hirnantian. There was then a rapid rise in sea level accompanied by widespread transgresSion during the Glyptograptus persculptus Biozone. Areas which had remained marine during most of the Hirnantian were mainly blanketed by black anoxic graptolitic shales. Only locally, as at Meifod in the east part of the Welsh Basin, do neritic deposits with a mixed trilobite/brachiopod fauna succeed shallow marine Hirnantian rocks. PJB R e
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e
s
References not included here are cited in Williams et al. (1972). ALLEN, P. M., JACKSON,A. A. & RUSHTON,A. W. A. 1981. The stratigraphy of the Mawddacb Group in the Cambrian succession of North Wales. Proceedings of the Yorkshire Geological Society, 43, 295-329. ARTHURTON,R. S., JOHNSON,E. W. & MUNDV,D. J. C. 1988. Geology of the country around Settle. Memoir of the British Geological Survey. Sheet 60 (England & Wales). HMSO. BARNESC. R., NORFORD, B. S. & SKEVINGTON,D. 1981. The Ordovician System in Canada. Correlation Chart and Explanatory Text. International Union of Geological Sciences, Publication 8. BARRETT, T. J., JENKYNS, H. C., LEGGETT,J. K. & ROBERTSON,A. H. F. 1982. Comment and reply on 'Age and origin of Ballantrae ophiolite and its significance to the Caledonian orogeny and the Ordovician time scale'. Geology, 9, 331-333. BASSET'r, M. G. 1981. The Ordovician Brachiopods of Cornwall. Geological Magazine, 118, 647-664. BECKLY,A. J. 1987. Basin development in North Wales during the Arenig. Geological Journal, 22, 19-30. 1988. The stratigraphy of the Arenig Series in the Aberdaron to Sam area, western Llr North Wales. Geological Journal, 23, 321-337. BERGSTR6M, S. M. & ORCHARD, M. J. 1985. Conodonts of the Cambrian and Ordovician Systems from the British Isles. In: HIGGINS,A. C. & AUSTIN,R. U (eds) A stratigraphical index of conodonts. Ellis Horwood, Chichester, 32-67, Table I (238-239). , RHODES, F. H. T., & LINDSTR6M,M. 1987. Conodont biostratigraphy of the Llanvirn-Llandeiloand Llandeilo-Caradoc Series boundaries in the Ordovician System of Wales and the Welsh Borderland. In: AUSTIN,R. L. (ed.) Conodonts: Investigative techniques and applications, Ellis Horwood, Chichester, 284-315. BEVINS,R. E. 1982. Petrology and geochemistry of the Fishguard Volcanic Complex, Wales. Geological Journal, 17, 1-21. BLUCg, B. J. 1983. R61e of the Midland Valley of Scotland in the Caledonian Orogeny. Transactions of the Royal Society of Edinburgh: Earth Sciences, 74, 119-136. -1985. The Scottish paractectonic Caledonides. Scottish Journal of Geology, 21, 437-464. , INGHAM,J. K., CURRY, G. B. & WILLIAMS,A. 1984. VI. Stratigraphy and tectonic setting of the Highland Border Complex. In: CURRY, G. B., BLUCK, B. J., BURTON,C. J., IrqGHAM,J. K., SIVETER,D. J. & WILLIAMS,A. (eds) Age, evolution and tectonic history of the Highland Border Complex, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 75, 113-133. & LEAgE, B. E. 1986. Late Ordovician to Early Silurian amalgamation of the Dalradian and adjacent Ordovician rocks in the British Isles. Geology, 14, 917919. BRANNEY,M. J. & SOPER,N. J. 1988. Ordovician volcano-tectonics in the English Lake District. Journal of the Geological Society, London, 145, 367-376. BRENCHLEY, P. J., HARPER, J. C., MITCHELL, W. I. & ROMANO, M. 1977. A reappraisal of some Ordovician successions in eastern Ireland. Proceedings of the Royal Irish Academy, 77, section B, 65-85~ - - & NEWALL,G. 1984. Late Ordovician environmentalchanges and their effect on faunas. In: BRUTON,D. L. (ed.) Aspects of the Ordovician System. Palaeontological Contributions from the University of Oslo, No. 295, 65-79. & PICKERILL, R. K. 1980. Shallow sub-tidal sediments of Soudleyan (Caradoc) age in the Berwyn Hills, North Wales, and their palaeogeographical context. Proceedings of the Geologists' Association, 91, 177-174. BROCK, P. M., COLTHURST,J. R., FEELEY,M., GARD1NER,P. R. R., PENNEY,S. R., REEVES,T. J., SHANNON,P. M., SMITH,D. G. & VANGUESTAINE,M. 1979. Southeast Ireland: Lower Palaeozoic stratigraphy and depositional history. In: HARRIS, A. L., HOLLAND,C. H. & LEAKE, B. E. (eds) The Caledonides of the British Isles--reviewed. Geological Society, London, Special Publication, 8, 533-544. BULMAN,O. M. B. & RUSHTON,A. W. A. 1973. Tremadoc faunas from boreholes in Central England. Bulletin of the Geological Survey of Great Britain. 43, 1-40. BURTON,C. J., HOCKEN,C., MACCALLUM,D. & YOUNG,M. E. 1984. Chitinozoa and the age of the Margie Limestone of the North Esk. Proceedings of the Geologice,/ Society of Glasgow, 124/125, 27-32.
-
-
-
-
-
-
CAMPBELL,S. D. G., HOWELLS,M. F., SMITH,M. & REEl)MAN,A. J. 1988. A Caradoc failed-rift within the Ordovician marginal basin of Wales. Geological Magazine, 125, 257-266. CARLISLE,H. 1979. Ordovician stratigraphy of the Tramore area, County Waterford, with a revised Ordovician correlation for south-east Ireland. In: HARRIS,A. L., HOLLAND, C. H. & LEAKE, B. E. (eds) The Caledonides of the British Isles-reviewed. Geological Society, London, Special Publication, 8, 545-554. CAVE, R. 1977. Geology of the Malmsbury District. Memoir of the Geological Survey of Great Britain, Sheet 251 (England & Wales). HMSO. CAWOOD, P. A., WILLIAMS,H., O'BRIEN, S. J. & O'NEILL, P. P. 1988. Field Trip Guidebook. Trip A1. A geologic cross-section of the Appalachian orogen. Geological Association of Canada, Mineralogical Association of Canada & Canadian Society of Petroleum Geologists. St John's, Newfoundland. Coc~s, L. R. M. 1985. The Ordovician-Silurian boundary. Episodes, 8, 98-100. & FORTEY, R. A. 1982. Faunal evidence for oceanic separations in the Palaeozoic of Britain. Journal of the Geological Society, London, 139, 465--478. & -1988. Lower Palaeozoic facies and faunas around Gondwana. In: AUDLEY-CHARLES,M. G. & HALLAM,A. (eds) Gondwana and Tethys. Geological Society, London, Special Publication, 37, 183-200. & RICr,AgDS, R. B. (eds). 1988..6 global analysis of the Ordovician-Silurian boundary. Bulletin of the British Museum of Natural History (Geology), 43,
-
-
-
-
-
-
! - 3 9 4 .
COOPER,A. H. & MOLYNEUX,S. G. 1990. The age and correlation of the Skiddaw Group (early Ordovician) sediments in the Cross Fell inlier (northern England). Geological Magazine, 127, 147-157. COWIE, J. W., RUSHTON,A. W. A. & STOBBLEraEtD,C. J. 1972. A correlation of Cambrian rocks in the British Isles. Geological Society London, Special Report, 2. CURRY, G. B., INGHAM,J. K., BLUCK,B. J. & WILLIAMS,A. 1982. The significance of a reliable Ordovician age for some Highland Border rocks in Central Scotland. Journal of the Geological Society, London, 139, 451-454. -& WILLIAMS,A. 1984. Lower Ordovician brachiopods from the Ben Suardal Limestone Formation (Durness Group) of Skye, western Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 75, 301-310. DEMPSTER, T. J. 1985. Uplift patterns and orogenic evolution in the Scottish Dalradian. Journal of the Geological Society, London, 142, 111-128. & BLUCK, B. J. 1989. The age and origin of boulders in the Highland Border Complex: constraints on terrane movements. Journal of the Geological Society, London, 146, 377-379. DUNNING, G. R. & KROGH, T. E. 1985. Geochronology of ophiolites of the Newfoundland Appalachians. Canadian Journal of Earth Sciences, 22, 1659-1670. FIRMAN, R. J. & LEE, M. K. 1986. Age and structure of the concealed English Lake District batholith and its probable influence on subsequent sedimentation, tectonics and mineralisation. In: NESBITT,R. W. & NICnOL, I. (eds) Geology' in the real world--the Kingsley Dunham volume, 117-127. The Institution of Mining and Metallurgy. FORTEY, R. A. 1975. The Ordovician Trilobites of Spitsbergen. II. Asaph~dae, Nileidae, Raphiophoridae and Telephinidae of the Valhallfonna Formation. Norsk Polarinstitutt Skrifter, 162, 1-207. 1984. Global earlier Ordovician transgressions and regressions and their biological implications. In BRUTON,D. L. (ed.) Aspects of the Ordovician System. Palaeontological contributions from the University of Oslo, No. 295, 37-50. & OWENS, R. M. 1978. Early Ordovician (Arenig) stratigraphy and faunas of the Carmarthen district, south-west Wales. Bulletin of the British Museum of Natural History (Geology), 30, 225-294. & -1987. The Arenig Series in South Wales. Bulletin of the British Museum of Natural History (Geology), 41, 69-307. - - , - - & RUSHTOtq,A. W. A. 1989. The palaeogeographic position of the Lake District in the earlier Ordovician. Geological Magazine, 126, 9-17. GOLDRING, STEPI-mqSOS,D. G. 1972. The depositional environment of three Starfish Beds. Neues Jahrbuch fiir Geologie und Palaontologie Mittheilungen, 10, 611--624. GRAHAM, J. R. & SMITH, D. G. 1981. The age and significance of a small Lower Palaeozoic inlier in County Mayo. Journal of Earth Sciences, Royal Dublin -
-
-
-
-
-
R .
&
Society. 4, I-6. HAMILTON, P. J., BLUCg, B. J. & HALLIDAY,A. N. 1984. Sm-Nd from Ballantrae complex, SW Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 75, 183-187. HARPER, D. A. T. 1979. The environmental significance of some faunal changes in the Upper Ardmillan succession (upper Ordovician), Girvan, Scotland. In: HARRIS, A. L., HOLLAND, C. H. & LEAKE, B. E. (eds) The Caledonides of the British Isles--reviewed. Geological Society, London, Special Publication, 8, 439. 1982a. The late Ordovician Lady Burn Starfish Beds of the Girvan District. Proceedings of the Geological Society of Glasgow, 122/123, 28. 1982b. The stratigraphy of the Drummuck Group (Ashgill), Girvan. Geological Journal, 17, 251-277. 1984. Brachiopods from the Upper Ardmillan succession (Ordovician) of the Girvan district, Scotland. Part 1. Monograph of the Palaeontographical Society, London, 1-78. - - , GRAHAM,J. R., OWEN, A. W. & DONOVAN,S. K. 1988. An Ordovician fauna from Lough Shee, Partry Mountains, Co. Mayo, Ireland. Geological Journal, 23, 293-310. & PARKES, M. A. 1989. Short Paper: Palaeontological constraints on the definition and development of Irish Caledonide Terranes. Journal of the Geological Society, London, 146, 413-415. - - , WILLIAMS,D. M. & ARMSTRONG,H. A. 1989. Short Paper: Stratigraphical correlations adjacent to the Highland Boundary fault in the west of Ireland. Journal of the Geological Society, London, 146, 381-384. HAUGHTON,P. D. W. 1988. A cryptic Caledonian flysch terrane in Scotland. Journal of the Geological Society, London, 145, 685-703. HENRY, J.-L. 1980. Trilobites ordoviciens du Massif Armoricain. Mdmoires de la Socidtd Gbologique et Mineralogique de Bretagne, 22, 1-250. HOLLAND, C. H. (ed.) 1981. A Geology of Ireland. Scottish Academic Press, Edinburgh. HOWELLS, M. F., CAMPBELL,S. D. G., REEDMAN,A. J. & TUNNICLIFF,S. P. 1987. A fissure-controlled acidic volcanic centre (Ordovician) at Yr Arddu, North Wales. Geological Journal, 22, 133-150. - - ' , LEVERIDGE,B. E., ADDISON,R. & REEDMAN,A. J. 1983. The lithostratigraphical subdivisions of the Ordovician underlying the Snowdon and Crafnant Volcanic Groups, North Wales. Report of the Institute of Geological Sciences, 83/I, 11-15. , EVANS,C. D. R. & NUTT, M. J. C. 1981. Dolgarrog: D'escription of 1. "25000 Geological Sheet SH 76. Classical areas of British Geology, Institute of Geological Sciences. HMSO, London. -
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CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
36
ORDOVICIAN
, REEDMAN,A. J. & LEVERIDGE,B. E. 1985. Geology of the country around Bangor. Explanation of the 1.'50000 sheet. British Geological Survey, Sheet 106, England and Wales. HUGHES, R. 1989. Llandeilo and Caradoc graptolites of the Builth and Shelve inliers. Monograph of the Palaeontographical Society, 1-89 (Publ. No. 577). HURST, J. M. 1979a. The environment of deposition of the Caradoc Alternata Limestone and contiguous deposits of Salop. Geological Journal, 14, 15-40. - - 1979b. The stratigraphy and brachiopods of the upper part of the type Caradoc of south Salop. Bulletin of the British Museum of Natural History (Geology), 32, 183-304. HUTTON,D. H. W., AFTALION,M. & HALLIDAY,A. N. 1985. An Ordovician ophiolite in County Tyrone, Ireland. Nature, 315, 210-212. INCE, D. 1984. Sedimentation and tectonism in the Middle Ordovician of the Girvan district, SW Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 75, 225-237. INGHAM,J. K. 1978. Geology of a continental margin 2: middle and late Ordovician transgression, Girvan. In: BowES, D. R. & LEAKE,B. E. (eds) Crustal evolution in northwestern Britain and adjacent regions. Geological Journal Special Issue 10, 163-176. - - , CURRY, G. B. & WILLIAMS,A. 1986. Early Ordovician Dounans Limestone fauna, Highland Border Complex, Scotland. Transactions of the Royal Society of Edinburgh." Earth Sciences, 76 (4) (for 1985). --, MCNAMARA, J. K. & RICKARDS, R. B. 1978. The upper Ordovician and Silurian rocks. In: MOSELEY,F. (ed.) The Geology of the Lake District. Yorkshire Geological Society, Occasional Publication, 3, 121-145. & TRIPe, R. P. 1991. The trilobite fauna of the Middle Ordovician Doularg Formation of the Girvan district, Scotland, and its palaeoenvironmental significance. Transactions of the Royal Society of Edinburgh: Earth Sciences, 82, 27-54. JACKSON,D. E. 1978. The Skiddaw Group. In: MOSELEY,F. (ed.) The Geology of the Lake District. Yorkshire Geological Society, Occasional Publication, 3, 79-98. JAMES, D. M. D. 1975. Caradoc turbidites at Poppit Sands (Pembrokeshire), Wales. Geological Magazine, 112, 295-304. 1985. Relative sea-level movements, palaeohorizontals and the depositional relationships of Upper Ordovician sediments between Corris and Bala, midWales. Mercian Geologist, 10, 19-26. 1987. Ashgill sediments between Plynlimon and Machynlleth, west central Wales. Mercian Geologist, 10, 265--279. JENKINS, C. J. 1983. Ordovician graptolites from the Great Paxton Borehole, Cambridgeshire. Palaeontology, 26, 641~553. KELLEY, S. & BLUCK, B. J. 1989. Short Paper: Detrital mineral ages from the Southern Uplands using *~ laser probe. Journal of the Geological Society, London. 146, 401-403. KNEELER, B. C. & AFTALION, M. 1987. The isotopic and structural age of the Aberdeen Granite. Journal of the Geological Society, London, 144, 717-721. KOKELAAR,B. P. 1979. Tremadoc to Llanvirn vulcanism on the southeast side of the Harlech Dome (Rhobell Fawr), N Wales. In. HARRIS,A. L., HOLLAND,C. H. & LEAKE, B. E. (eds) The Caledonides of the British Isles--reviewed. Geological Society, London, Special Publication, 8, 591-596. 1988. Tectonic controls of Ordovician arc and marginal basin volcanism in Wales. Journal of the Geological Society, London, 145, 759-775. - - - , BEVINS, R. E. & ROACH, R. A. 1985. Submarine silicic volcanism and associated sedimentary processes: a case study on Ramsey Island, S.W. Wales. Journal of the Geological Society, London, 142, 591-613. , HOWELLS, M. F., BEVINS, R. E. & DUNKLEY, P. N. 1984. The Ordovician marginal basin of Wales. In: KOKELAAR,B. P. & HOWELLS,M. F. (eds) Marginal Basin Geology. Geolog;cal Society, London, Special Publication, 16, 245-269. LEAKE, B. E., TANNER, P. W. G., SINGH, D. & HALLIDAY, A. N. 1983. Major southward thrusting of the Dalradian rocks of Connemara, western Ireland. Nature, 305, 210-213. LEAT, P. T. & THORPE, R. S. 1986. Geochemistry of an Ordovician basalt-trachybasalt-subalkaline/peralkaline rhyolite association from the Lleyn Peninsula, North Wales, U.K. Geological Journal, 21, 29-43. LEGGETT,J. K., MCKERROW,W. S. & CASEY,D. M. 1982. The anatomy of a Lower Palaeozoic accretionary forearc: the Southern Uplands of Scotland. In: LEGGETT, J. K. (ed.) Trench-Forearc Geology. Geological Society, London, Special Publication, 10, 495-520. LOCKLEY, M. G. 1983. A review of brachiopod dominated palaeocommunities from the type Ordovician. Palaeontology, 26, 111-145. LOWMAN,R. D. W. & BLOXAM,T. W. 1981. The petrology of the Lower Palaeozoic Fishguard Volcanic Group, and associated rocks E of Fishguard, N Pembrokeshire (Dyfed), South Wales. Journal of the Geological Society, London, 138, 47-68. LYNAS, B. D. T. 1973. The Cambrian and Ordovician rocks of the Migneint area, North Wales. Journal of the Geological Society, London, 129, 481-503. -1983. Two new Ordovician volcanic centres in the Shelve inlier, Powys, Wales. Geological Magazine, 120, 535-542. MCKERROW, W. S., LAMBERT,R. St. J. & COCKS, L. R. M. 1985. The Ordovician, Silurian and Devonian Periods. In: SNELHNG,N. J. (ed.) The Chronology of the Geological Record. Geological Society, London, Memoir, 10, 73-80. , LEGGETT,J. K. & EALES, M. 1977. Imbricate thrust model of the Southern Uplands of Scotland. Nature, 267, 237-239. & SOPER, N. J. 1989. The Iapetus suture in the British Isles. Geological Magazine, 126, 1-8. MAX, M. D., BARBER,A. J. & MARTINEZ,J. 1990. Terrane assemblage of the Leinster Massif, SE Ireland, during the Lower Palaeozoic. Journal of the Geological Society, London, 147, 1035-1050. MOLYNEUX, S. G. 1979. New evidence for the age of the Manx Group, Isle of Man. In: HARRIS,A. L., HOLLAND,C. H. & LEAKE,B. E. (eds) The Caledonides of the British Isles--reviewed. Geological Society, London, Special Publication, 8, 415421. 1988. Micropalaeontological evidence for the age of the Borrowdale Volcanic Group. Geological Magazine, 125, 541-542. & RUSHTON,A. W. A. 1988. The age of the Watch Hill Grits (Ordovician), English Lake District: structural and palaeogeographical implications. Transactions of the Royal Society of Edinburgh." Earth Sciences, 79, 43~9. MORRIS, J. H. 1987. The Northern Belt of the Longford-Down Inlier, Ireland and Southern Uplands, Scotland: an Ordovician back-arc basin. Journal of the Geological Society, London, 144, 773-786. MURPHY, F. C. 1987. Evidence for late Ordovician amalgamation of volcanogenic terranes in the Iapetus suture zone, eastern Ireland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 78, 153-167. OLD, R., SUMBLER, M. G. & AMBROSE, K. 1987. Geology of the country around Warwick. Memoir of the British Geological Survey, Sheet 184 (England & Wales). HMSO, London.
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PANKHURST,R. J. 1970. The geochronology of the basic igneous complexes. Scottish Journal of Geology, 6, 83-107. -1974. Rb-Sr whole rock chronology of (~aledonian events in northeastern Scotland. Bulletin of the Geological SocieO' of America, 85, 345-350. PHARAOH, T. C., MERRIMAN,R. J., WEBB, P. C. & BECKINSALE,R. D. 1987. The concealed Caledonides of eastern England: preliminary results of a multidisciplinary study. Proceedings of the Yorkshire Geological Society, 46, 355-369. , EVANS,J. A., BREWER,T. S., WEBB,P. C. & SMrrH, N. J. P. 1991. Early Palaeozoic arc-related volcanism in the concealed Caledonides of southern Britain. Annuales de la Socibtd Gbologique de Beige. (Brussels Symposium Volume) in press. PIPER, J. D. A. 1978. Palaeomagnetism and palaeogeography of the Southern Uplands block in Ordovician times. Scottish Journal of Geology. 14, 93-107. PRICE, D. 1980. The Ordovician trilobite fauna of the Sholeshook Limestone Formation of South Wales. Palaeontology, 23, 839-887. PUDSEY,C. J. 1984a. Ordovician stratigraphy and sedimentology of the South Mayo inlier. Irish Journal Earth Science, 6, 15-45. 1984b. Fluvial-marine transition in the Ordovician of Ireland--a humid regional fan delta? Geological Journal, 19, 143-172. ROMANO,M. 1980. The stratigraphy of the Ordovician rocks between Slane (County Meath) and Collon (County Louth), eastern Ireland. Journal of Earth Sciences, Royal Dublin Society, 3, 53-79. Ross, R. J. Jr et al. 1982. The Ordovician System in the United States. Correlation chart and explanatory notes. International Union of Geological Sciences, Publication 12. RUSHTON, A. W. A. 1985. A Lancefieldian graptolite from the Lake District. Geological Magazine, 122, 329-333. & HUGHES. 1981. Ordovician trilobite fauna of the Great Paxton Borehole, Cambridgeshire. Geological Magazine, 118, 623-646. , STONE,P., SMELLIE,J. L. & TUNNICLIEF,S. P. 1986. An early Arenig age for the Pinbain sequence of the Ballantrae Complex. Scottish Journal of Geology, 22, 41-54. & TRIPP, R. P. 1979. A fossiliferous lower Canadian (Tremadoc) boulder from the Benan Conglomerate of the Girvan district. Scottish Journal of Geology, 15, 321-327. RYAN, P. D., FLOYD, P. A. & ARCHER, J. B. 1980. The stratigraphy and petrochemistry of the Lough Nafooey Group (Tremadocian), western Ireland. Journal of the Geological Society, London, 137, 443-458. SADLER, P. M. 1974. Trilobites from the Gorran Quartzites of south Cornwall. Palaeontology, 17, 71-93. SKEVINGTON, D. 1974. Controls influencing the composition and distribution of Ordovician graptolite faunal provinces. In: RICKARDS,R. B., JACKSON,O. E. & HUGHES, C. P. (eds) Graptolite studies in honour of O. M. B. Bulman. Special Papers in Palaeontology, 13, 59-73. SMITH, D. G. 1981. Progress in Irish Lower Palaeozoic palynology. Review of Palaeob~tany and Palynology, 34, 137-148. STONE,P., FLOYD,J. D., BARNES,R. P. & LINTERN,B. C. 1987. A sequential back-arc and foreland basin thrust duplex model for the Southern Uplands of Scotland. Journal of the Geological Society, London, 144, 753-764. & STRACHAN, I. 1981. A fossiliferous borehole section within the Ballantrae ophiolite. Nature, 293, 455456. STYLES, M. T., STONE, P. & FLOYD, J. D. 1989. Short Paper: Arc detritus in the Southern Uplands: mineralogical characterization of a 'missing' terrane. Journal of the Geological Society, London, 146, 397-400. TANNER, P. W. G., DEMPSTER,T. J. & DICKIN, A. P. 1989. Short Paper: Time of docking of the Connemara terrane with the Delaney Dome Formation, western Ireland. Journal of the Geological Society, London, 146, 389-392. THIRLWALL,M. F. & BLUCK,B. J. 1984. Sm-Nd isotope and chemical evidence that the Ballantrae "ophiolite", S.W. Scotland is polygenetic. In: GLASS, I. G., LIPPARD, S. J. & SHELTON, A. W. (eds). Ophiolites and Oceanic Lithosphere. Geological Society, London, Special Publication, 13, 215-230. -& FITTON, J. G. 1983. Sm-Nd garnet age for the Ordovician Borrowdale Group, English Lake District. Journal of the Geological Society, London, 140, 511-518. THOMAS,P. R. 1979. New evidence for a Central Highland Root Zone. In: HARRIS, A. L., HOLLAND, C. H. & LEAKE, B. E. (eds) The Caledonides of the British Isles--reviewed. Geological Society, London, Special Publication, 8, 205-211. TRAYNOR, J.-J. 1988. The Arenig in South Wales: sedimentary and volcanic processes during the initiation of a marginal basin. Geological Journal, 23, 275-292. TRENCH, A., BLUCK, B. J. & WATTS,D. R. 1988. Palaeomagnetic studies within the Ballantrae ophiolite. Earth and Planetary Science Letters, 90, 431-440. TRIPP, R. P. 1976. Trilobites from the basal Superstes Mudstones (Ordovician) at Aldons Quarry, near Girvan, Ayrshire. Transactions of the Royal Society of Edinburgh. Earth Sciences, 69, 369-423. 1979. Trilobites from the Ordovician Auchensoul and Stinehar Limestones of the Girvan District, Strathclyde. Palaeontology, 22, 37-40. -1980. Trilobites from the Ordovician Ardwell Group of the Craighead Inlier, Girvan district, Scotland. Transactiohs of the Royal Society of Edinburgh: Earth Sciences, 71, 147-157. VANGUESTAINE, M. 1977. Donnres palynologiques nouvelles dans l'Ordovicien infrrieur du bassin de la Senne, massif du Brabant, Belgique. Annales de la Sociktd Gbologique de Beige, 100, 193-198. WADGE, A. J. 1978. Classification and stratigraphical relationships of the Lower Ordovician rocks. In: MOSELEY, F. (ed.) The Geology of the Lake District. Yorkshire Geological Society, Occasional Publication No. 3, 68-78. WHITTARD,W. F. 1979. An account of the Ordovician rocks of the Shelve Inlier in west Salop and part of north Powys (ed. DEAN, W. T.). Bulletin of the British Museum of Natural History (Geology), 33, 1-69. WHITTINGTON,H. B., DEAN, W. T., FORTEY, R. A., RICKARDS.R. B., RUSHTON, A. W. A. & WRIGHT,A. D. 1984. Definition of the Tremadoc Series and the Series of the Ordovician System in Britain. Geological Magazine, 121, 17-33. WILLIAMS, A. & CURRY, G. B. 1985. Lower Ordovician Brachiopoda from the Tourmakeady Limestone, Co. Mayo, Ireland. Bulletin of the British Museum (Natural History) Geology, 311, 183-269. , STRACHAN,I., BAssETT,D. A., DEAN'W. T., INGHAM,J. K., WRIGHT,A. D. & WHITTINGTON,H. B. 1972. A Correlation of Ordovician rocks in the British Isles. Geological Society, London, Special Report, 3. WILLIAMS,D. M., ARMSTRONG,H. A. & HARPER,D. A. T. 1988. The age of the South Connemara Group, Ireland and its relationship to the Southern Uplands Zone of Scotland and Ireland. Scottish Journal of Geology, 24, 279-287. ZALASIEWICZ, J. 1984. A re-examination of the type Arenig Series. Geological Journal, 19, 105-124. -
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Devonian B. J. B L U C K , J . C. W . C O P E
The Devonian System was the first pre-Quaternary system to have its base established at an internationally ratified Global Stratotype Section and Point (GSSP). The base of the System was defined at the base of the Monograptus uniformis Biozone at Klonk in Czechoslovakia (McLaren 1977). The upper boundary of the System is fixed by the base of the Carboniferous, which has been recently ratified in a GSSP at La Serre, H+rault, France, at a point coincident with the first appearance of the conodont Siphonodella sulcata. Series. GSSPs have now been designated to define the bases of the Middle and Upper Devonian Series (that of the Lower Devonian being automatically defined by the System boundary). Stages. Consequent upon the selection of a Czech type section for the basal boundary stratotype of the System, the Germanic stages for the lowest parts of the Devonian were no longer appropriate and Lochkovian and Pragian stages have now been formally defined with stratotypes ratified by the I.U.G.S. Devon, the nomenclatorial type locality for the Devonian System (Sedgwick & Murchison 1839) is a region of great tectonic complexity and has not provided suitable sections on which to base international corrglations. However, the rocks are frequently richly fossilferous and firm correlations can now be established with the intensively studied developments of the Devonian in Belgium and Germany. Northwards from Devon are found large tracts of the predominantly fluviatile and lacustrine facies which characterizes the Old Red Sandstone. Correlation between the marine Devonian rocks of southwest England and the Old Red Sandstone facies to the north still presents major problems, but the growing use of miospores is proving a useful tool in correlation. In some of the more highly oxidized horizons of the Old Red Sandstone, however, spores are hard to find and correlations consequently less satisfactory. Because of these problems, and the rapidly evolving tectonic controls, the Devonian maps, rather than representing 'snapshots' of Devonian time, represent 'time slices' taken at intervals of approximately 10 Ma,' thus allowing some degree of latitude in correlation of the marine Devonian, for which sound biostratigraphical schemes exist (for areas which have yielded faunas) with the Old Red Sandstone facies. JCWC
DI: L o c h k o v i a n Following the designation of the base of the Devonian System at Klonk in Czechoslovakia, the Lochkovian Stage has been internationally accepted as the basal Devonian stage. A general equivalence is taken here between earlier usage of Gedinnian, Siegenian and Emsian, and Lochkovian, Pragian and Emsian of current usage. Lower Devonian sequences in southwest England are poorly dated. Except in south Cornwall, no evidence for Lochkovian has been found in southwest England. Historically, interpretations of the ground in south Cornwall have varied considerably. More recently, the early Devonian rocks there have been regarded as part of a coherent stratigraphical sequence (the Roseland Volcanics Unit) resting on Llandeilo quartzites and thrust northwards as an imbricate stack into their present position (Saddler 1973; Matthews in House et al. 1977). Subsequent work, however, has interpreted these Devonian carbonates and all the Lower Palaeozoic material to be phacoids in an olistostrome of Givetian and Frasnian age (Barnes 1983; Barnes & Andrews 1986; Holder & Leveridge 1986). This material is thought to have been derived from a cratonic rise to the south bearing a Devonian carbonate-rich sequence of Lochkovian to at least Givetian age. Further north in Cornubia, the oldest Devonian rocks are assigned to the Dartmouth Slate, which crops out from the mid-west Cornish coast to the Devon coast south of Tor Bay. The formation consists predominantly of argillites of continental aspect, with some volcanics, in which the earliest dates are given by fish of probable early Pragian age (House & Selwood 1966; Orchard in House et al. 1977), although the slates may extend down into the Lochkovian (House 1975). What lies beneath the Dartmouth Slate is not known, but Cope & Bassett (1987) and Cope (1988) have suggested that it could well have been the eroded Pretannian metamorphic basement. The Siluro-Devonian boundary in Wales and the Welsh Borderland falls within the lower part of the Old Red Sandstone facies. Traditionally the boundary was placed a t the junction of the Ludlow Series with the basal member of the overlying Downton Group~the Ludlow Bone Bed. More recently, however, since the international ratification of the basal Devonian stratotype point, it has been realized that the boundary must correlate with a much higher position in the British succession, and that the Pfidoli Series of the Silurian correlates approximately with the Downton 'Series' of Wales 9 1992The Geological.Society
& C. T. S C R U T T O N
and the Welsh Borderland. This being the case, the base of the Devonian correlates approximately with the Downton Group/Ditton Group boundary. This latter boundary corresponds with a facies change from thick brickred mudstones to one of fining-upwards cycles from conglomerates, through sandstones to dark red mudstones in which calcretes are frequently developed. This facies change may well be diachronous, although at the junction there are developed, over much of the area, a particularly thick sequence of pedogenic carbonates, the Psammosteus Limestones. Such thick calcrete horizons as the Psammosteus Limestones probably represent a considerable period of geomorphological stability lasting for the order of 104 years (Allen 1985) and are thus likely to be of use in chronostratigraphical subdivision. Providing greater precision in correlation, however, is the event-stratigraphical correlation afforded by the Townsend Tuff Bed, a sequence of three superimposed tufts, described initially from southwest Wales by Allen & Williams (1978) but subsequently discovered by these workers over a wide area of South Wales and northwards to Shropshire (Allen & Williams 1981). In this latter work, the Townsend Tuff Bed was formally proposed as the basal Devonian marker over the South Wales and Welsh Borderlands area, but biostratigraphical correlations based on spores (Richardson & McGregor 1986) make it clear that the correlative of the Pfidoli/Lochkovian boundary in the Old Red Sandstone lies above the Psammosteus Limestone horizon. Thus the Townsend Tuff Bed must be of approximately mid Pfidoli in age. The younger Pickard Bay Tuff Band (Allen & Williams 1982) must lie quite close to the Silurian/Devonian boundary. Above the Psammosteus Limestones, the Ditton Group beds of the Old Red Sandstone exhibit a fundamental mineralogical change; they are poorly micaceous when compared with the underlying Downton Beds, and their heavy mineral suite is no longer dominated by garnet and other minerals indicating a regionally metamorphosed source. Instead, the rocks are rich in variously sized clasts and mineral suites indicating a predominance of igneous and sedimentary sources. The sudden and pronounced change in depositional style, mineralogy and clast type led Allen (1985) to suggest that the Psammosteus Limestones indicate the initiation of the Devonian uplift of the Irish Sea/Wales area resulting in major changes to the fluvial systems from a northern source area which had developed during the Pfidoli Epoch
(q.v.). Over the West Midlands the Lochkovian is again represented by the lower part of the Ditton Group, dominated by fining-upwards cyclothems and yielding rich faunas of acanthodians, arthrodires, heterostracans and osteostracans (Ball & Dineley 1961). The St. Maugham's Group in southwest Wales and the Forest of Dean correlates with this horizon, as does the upper part of the undivided Red Marl Group of the Geological Survey's early work in central south and southwest Wales. The highest parts of the Old Red Sandstone of eastern Anglesey (Allen 1965) may be of this age. Palaeocurrent indications for the Lochkovian part of the Old Red Sandstone facies, throughout Wales and the Welsh Borderland region show that the fluviatile sediments came entirely from a northerly direction. In the north of Britain, basins initiated in Pfidoli times continued to receive sediment during the Lochkovian. As with many basins of this type they progressively expanded so that younger sediments overlap older to rest on basement. The basement uplift which characterized the mid Pfidoli continued into the Lochkovian, with the result that northern Scotland and areas along the Great Glen Fault continued to be sources for sediments. However, as there is a climax of uplift ages at c. 420 Ma, the Lochkovian can be seen as a time of declining relief. In the Southern Uplands the Loch Doon pluton and the Priestlaw intrusion cooled at this time and both may have been a source of lavas. Conglomerates occur in isolated outliers within the northeast of the region; they are cut by dykes of 400 Ma and may therefore be of Lochkovian or Pfidoli age. They rest on an eroded surface of greywackes, implying that by this time the northeastern part of the Southern Uplands was in, or near to, a peneplained condition. In the Midland Valley of Scotland, although much of the volcanic activity in the Ochil and Sidlaw Hills is known to be >410 Ma, there are lavas which interfinger with the Arbuthnott and Garvock groups which are of Lochkovian age or younger (Richardson et al. 1984). Another major volcanic centre was in the Pentland Hills where lavas interdigitate with Old Red Sandstone; their age is poorly constrained, but some are cut by felsites which, to the south at Tinto, yield an age of411 Ma (Thirlwall 1988). Some at least of this volcanism is likely to be of Lochkovian age. Lochkovian sediments occur widely on the northwest and southeast of the Midland Valley and on the north frequently rest unconformably on the Highland Border Complex. The conglomerates along most of the outcrop comprise mixtures of volcanic and metaquartzite clasts, and the sandstones 57
58
DEVONIAN
are mainly lithic arenites dominated by volcanogenic sediments. The Arbuthnott and L. Garvock groups overstep the Crawton Basin which is confined to the northeast Midland Valley in the region south of Stonehaven (Haughton & Bluck 1988). It is not clear whether the more widespread Lochkovian was deposited in one or more basins. There are places, particularly between L. Lomond and L. Venachar, where the Lower Old Red Sandstone is interpreted as having accumulated in a relatively small faultgenerated basin floored partly or wholly by Highland Border Complex. Here basal beds overlap each other southwestwards and this relationship, together with other criteria has been interpreted by Bluck (1990) as resulting from the sinistral migration of a fault which bounded a source region south and possibly north of the Highland Boundary Fault. To the north of this fault there are outliers of breccias, conglomerates and lava; some of these lie above the Lintrathern ignimbrite (415 Ma) and so may be Lochkovian. The lack of a significant contribution from the now adjacent Dalradian block, despite substantial subsidence during Old Red Sandstone times, may be explained by a combination of factors. The Dalradian surface may have been peneplained--in Lorne subhorizontal lavas rest on such a surface; elsewhere, calcretes are common in thin, sometimes fine-grained, sequences. Uplift ages of < 430 Ma are unknown from the Dalradian south of the steep zone. The Dalradian may also have been protected from erosion by a cover of lava and quartzite, but the Grampian Group and Moine Supergroup were still being uplifted and may be the source of the metaquartzite. It is also possible that the Dalradian block may have lain at some distance from the Midland Valley and has subsequently been brought in by thrust or other movement along the Highland Boundary Fault belt. On the southern boundary of the Midland Valley the sedimentary basins have sequences of lavas, lava conglomerates and lithic arenites. Here, as elsewhere within the Midland Valley, there was much contemporaneous erosion of lavas to yield rounded clasts. The bulk of the extrusive activity may again here be older than Lochkovian. It is clear from the Old Red Sandstone of the Midland Valley that the present bounding faults, the Southern Uplands and Highlands Boundary faults, exerted no recognizable influence on sedimentation. Bluck (1984) has suggested that they are late-stage features which cover ground where fractures of a more fundamental and probably quite different type occur. They are probably high-angle reverse faults which translated the Dalradian block to the southeast and the Southern Uplands to the northwest; the convergence may have taken place partly in Middle Old Red Sandstone times and partly in late Devonian-early Carboniferous times (cf. Kennedy 1958). The whole northern Scottish region is seen as a sediment source area during the Lochkovian. There is much evidence for intrusive igneous activity with some of the very large and high-level intrusions cooling at this time. In the Isles of Shetland the Graven Granite was cooled at 405 Ma; on the mainland the Grudie, Midgedale and Fearn granites all at c. 400 Ma. The Grampians were intruded with large granitoids (e.g. Etive, Rannoch Moor, Cairngorms) all of which probably had extensive lava flows associated with them. The Glen Coe cauldron subsidence and associated andesites and rhyolites have yielded ages of 421 Ma (Thirlwall 1988) but may well have extended into the Lochkovian. Although there is some degree of uncertainty over the age of the basal deposits in the region, there is meagre evidence to suggest that the marginal deposits around the southern fringes of the Orcadian Basin are younger than Lochkovian. In the west, the Lorne basin which was probably initiated in late Wenlock times may still have been receiving lavas, as the potential volcanic centres of Etive and Rannoch Moor were cooling. In Ireland, Lochkovian rocks may be present in the Cushendun-Cushendall area and in the Fintona Block. In both areas there is evidence from lavabearing conglomerates and possible lava flows for much andesitic volcanism. In the former area conglomerates and sandstone rest unconformably on the Dalradian. The conglomerates rich in quartzite clasts were dispersed from the northwest and are thought to fine southeastwards. Lava-rich conglomerates above have a source in the southeast and inter-tongue with axial, fluvial sediments which are dispersed to the northeast. Simon (1984a) has interpreted this sequence as having formed in a small basin with a northeastsouthwest axis. The Fintona Basin has a coarse fill from the NW which appears to interdigitate with fine sediments of alluvial plain and playa origin. The succession here includes rocks of younger Devonian age (q.v.). In the Curlew Mountains is a succession c. 1500 m thick including andesites and possible intrusive rocks (Simon 1984c). There are clear parallels with the Midland Valley of Scotland, but whether these rocks are Lochkovian or younger Lower Old Red Sandstone is not known. In the Dingle area, the Dingle Group may include at the top sediments of earliest Devonian age. BJB, JCWC, CTS
D2: Pragian-Emsian boundary In a belt across southwest England, from the Cornish coast to Tor Bay, the shallow marine facies of the Meadfoot Slate yields upper Pragian and Emsian shelly faunas (House et al. 1977; Selwood & Durrance 1982; Holder & Leveridge 1986). These argillaceous sediments with thin channel sands contain rarer limy horizons which are richly fossiliferous. Brachiopods, trilobites and bivalves are the main faunal elements with few solitary corals
and occasional Pleurodictyum. Horizons rich in the trace fossil Chondrites are characteristc. This facies succeeds the non-marine Dartmouth Slate in about mid-Pragian times and represents a marine transgression moving northwards across Cornubia. A clastic facies, the Staddon Grit locally succeeds or is laterally equivalent to the Meadfoot Slate. The Meadfoot Slate, as with the whole of the Devonian succession in the southern Cornubian outcrop belt, is involved in a series of northwardly transported thrust sheets. Shackleton et al. (1982) estimated the total shortening across the peninsula to be not less than 150 km, although Turner (1986) argued for a value of less than half this amount. Present outcrops must be considered to be largely allochthonous, which seriously complicates any detailed palaeogeographical reconstructions. Throughout this account, facies are described in their present geographical positions and palinspastic adjustment is confined to the generalized and speculative scheme in Fig. 1. South Wales sediment . . . . . . intermment
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Nappe emplacement active during Devonian,
obOucting Lizard ophiolite and Other oceanic rocks over olistostromes but beneath over-riding continental crust
Fig. 1. Generalized palinspastic reconstruction of the southwest England area in the Devonian. The purpose is to demonstrate the possible distribution of the main sedimentary basins and their main tectonic setting, ratherthandetailed lithofacies at any precise level within the Devonian. Not to scale. The oldest Devonian rocks of north Devon, the marine Lynton Slate, are late Emsian and/or early Eifelian age (Edmonds et al. 1985; Edmonds & Williams 1985) so that the northward extent of the transgression by the beginning of the Emsian is unknown. In south Cornwall, conodont faunas from limestone blocks in the Roseland Breccia Formation (Holder & Leveridge 1986) suggest the continuation of carbonate deposition across the Pragian-Emsian boundary (Sadler 1973) on a rise to the south, in the same tectonic setting as that for the Lochkovian. Over South Wales and the Welsh Borderland, the Pragian-Emsian boundary correlates very approximately with the boundary between the Ditton and Brecon groups of the Old Red Sandstone (House et al. 1977). In many areas of South Wales and the Forest of Dean the interval is represented by the Brownstones Formation--a sequence of fining-upward conglomerate-sandstone-mudstone cycles in which calcretes are poorly, if at all, developed. The succession coarsens upwards and mudstones become less well developed towards the top of the succession. Sandstones are the dominant members of the cycles and frequently form thick stacked channelfill sequences. The conglomerates are notable for containing abundant clasts derived from the Lower Palaeozoic rocks of the Welsh Basin, including Ordovician sediments, igneous and pyroclastic rocks and Silurian sediments. It seems clear that the Brownstones are more proximal than the underlying parts of the Old Red Sandstone and show clearly that uplift and erosion of the Welsh Basin to the north and west was proceeding as a prelude to the deformation episode which occurred during the Middle Devonian, The currents which transported this coarse sediment were evidently powerful and Allen (1985) pictured low-sinuosity systems of varying .depth over the coastal plain. He postulated a fall-line a few tens of kilometres to the northwest. The Brownstones are developed over an area extending from the Brecon Beacons eastwards to the Forest of Dean. They have been studied in detail by Tunbridge (1981) and Allen (1983). The Cosheston Group of southwest Wales shows a similar upward-coarsening succession, and has been studied by Thomas (1978). Across the whole area derivation of material seems generally to have been from the northwest, although locally other factors may have been important. Thus at the head of the Swansea Valley, movement along the Swansea Valley Fault allowed erosion of the nearby basement (Tunbridge 1980; Cope 1981) whilst to the southeast of there, in the Cardiff area, the Llanishen Conglomerate, lying locally beneath the Brownstones appears to be of southerly derivation (Allen 1975). The source of this material was suggested by Cope & Bassett (1987) to be from the faulted northern margin of the by-now largely obscured Pretannian block. In the Welsh Borderland exotic clasts of Welsh Basin material again dominate an upward-coarsening succession of mudstones, sandstones and
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
60
DEVONIAN
conglomerates which has been accorded various local names (Ball & Dineley 1952, 1961; Allen 1961; Greig et aL 1968). The Black Nore Sandstone of the Bristol area correlates satisfactorily lithostratigraphically with the Brownstones to the northwest, but is somewhat less coarse-grained and is thinner (Wallis 1928; Pick 1964). Two boreholes in southern England penetrated Pragian-early Emsian rocks. Spore dates of early Emsian were obtained from the Beckton and the Apley Barn boreholes (House et al. 1977). In Northern England, the Mell Fell Conglomerate of the area north of Shap has been often referred to the Devonian, since it is of Old Red Sandstone facies. The formation has been described by Kimber (1984) who concluded that the depositional r6gime was of stream-flooding in a mid-fan environment. There is still no indication of the age .of these rocks save that they are post-Silurian and earlier than the earliest marine Carboniferous of the area, but may therefore represent earliest Carboniferous initiation of sedimentation. They thus appear on the early Carboniferous maps (q.v.). In the Southern Uplands plutons were intruded at Cairnsmore of Fleet and Criffel-Dalbeattie and during this period the Cheviot Volcanic Complex was formed where the intrusives and associated lavas were generated simultaneously (395.9 Ma, Thirlwall 1988). The lavas, being the top of the igneous pile, were subject to erosion and are the remnants of a much more extensive series of flows. In view of this association of lavas and intrusions, it is probable that other granitic bodies are the intrusive equivalents of extensive lava flows which were associated with them at this time. There are fairly widespread lamprophyre dykes which are dated here at 395 Ma or older and may also be associated with plutons of that age (Thirlwall 1988). There is some doubt whether the basins along the north margin of the Midland Valley were separated at this time from those of the south. Indeed, since the Tinto felsite (c. 412 Ma) intrudes a great deal of the sequence in the southern Midland Valley there may be few or no rocks of this age present there. Most rocks ascribable to this interval in the Strathmore Syncline comprise sandstones and mudstones with some conglomerates; the latter are most abundant in the northeast and some earlier ones are known to be sourced in the south and SSW. Late in Emsian times, lateral sedimentation on the northwest margin of the Midland Valley introduced two local alluvial fans containing igneous and metamorphic (? Dalradian) clasts from the northwest (Haughton & Bluck 1988); this may be related to late thrusting along the Highland Boundary Fault, and to localized Dalradian uplift and development of the Strathmore Syncline. Fans of uncertain age built to the northwest along the southern margin of the Midland Valley and were sourced by volcanic rocks which covered the Southern Uplands or the ground subsequently replaced by them. In both the north and south margins of the Midland Valley finer-grained axial sediments were deposited, for at least part of the time, from a very large river system flowing from the northwest. In both areas there are thick sand bodies laid down by river systems, with bars up to 12 m thick in the Strathmore region, where there are also thick mudstones and siltstones (one > 700 m thick). The sandstones are rich in mica and lithic fragments some of which are volcanic, the thick siltstones and mudstones associated with the channel sands are probably overbank deposits. The rivers were evidently deep and may have been sourced in the Caledonian nappes of Norway and/ or Greenland where, in the Scandian event, there is evidence for substantial uplift at 410-380 Ma. The ground to the immediate south of the Great Glen Fault and to the north of L. Linnhe was undergoing uplift at this time, with K-Ar mica ages of 385 and 393 Ma respectively. Uplift ages down to 405 Ma occur in the northern Highlands and there are abundant Ar-Ar ages of c. 420 Ma along the eastern margin of the Moine Thrust zone (Kelley 1988). The blocking temperature of both these micas for both systems varies from 250-500~ so, with a geothermal gradient of 25~ from the time of the above age determinations there would be at least 15 km of rock to be removed from the surfaces whence the data were collected. Although it is not possible to specify the time interval over which the over-burden was removed, it is very probable that there was high ground over regions whence the dates were obtained and it should be regarded as a sediment source area. Many plutons were intruded during this period, particularly in the region of Rannoch Moor and Etive. There is a substantial difference between the age of parts of the Etive complex (401, 390 Ma) and the age of the Lorne lavas (423 Ma), so that the Etive complex is not likely to be the source of these lavas. However, the present outcrop of the Lorne lavas is only an eroded remnant of a former more extensive outcrop, part of which probably included younger lavas possibly related to the more deeply buried granites such as Etive. Thus lavas may have been present round the intrusions as shown on the map. In the northeast Highlands breccias and conglomerates formed alluvial fans as a response to the early development of the Orcadian Basin; dispersal was from the west and the fan composition reflects the source. The fans interfinger with red mudstones and siltstones which probably formed in playas which were later to become a larger and more permanent lake. Basins south of Helmsdale are also fringed on their western margins by alluvial fan breccias and conglomerates sourced in the local Moine rocks. These coarse sediments interfinger with fluvial deposits, some of which flow axially northwards (Dec 1988). Small basins also occur within the block but their age is uncertain; they contain mainly breccias, frequently in successions divided by internal unconformity. In the outcrops northwest of L. Linnhe deposition in a small basin is implied.
Most of the sedimentary basins on both the Moine and Dalradian basement have fills where the breccias comprise clasts of angular soft local lithologies (e.g. slate and schist), and rounded polycyclic durable lithologies such as quartzite and vein quartz. The combination may result from the repeated uplift and recycling of basins as found in fault zones (Bluck 1990). These basins may thus not be remnants of a former extensive cover, but be deeper parts of basins which happen to have been preserved; the ground immediately around them was probably undergoing contemporaneous uplift, as in the Lorne Basin and the basins along the Great Glen Fault (south of L. Ness and north of L. Linnhe) where uplift ages of 393 Ma are recorded. Similar basins in the east Grampian Highlands were probably sourced from the north. (c. 1 kin. of lacustrine clays and silts in the Moray Firth (Richards 1985), may represent an early development of the Orcadian lake (see D3)). In Ireland the rocks of the Cushendun-Cushendall area and the Fintona Block, described above (see also Simon 1984a.c) could well both extend up to this age. Indeed, the record of a pteraspid from the southwest corner of the Fintona Block (Harper & Hartley 1938) requires a Pragian-Emsian age. The Old Red Sandstone of the Curlew Mountains with its association of at least 1500 m of sediments with andesitic lavas and possible intrusives has clear parallels with the Midland Valley of Scotland, but its age has not been established. The small fault-bounded trough with a coarse clastic fill at Ballymastocker Bay, resting unconformably on the Dalradian, also has Scottish equivalents--again the age is uncertain. BJB, JCWC, CTS
D3: Givetian In South Devon limestone deposition was established on an E-W outer shelf rise in the Tor Bay area in the early Eifelian (Scrutton 1977a,b). Inland and northwest of Tor Bay it reached its maximum extent in the Givetian. On the coast, the Eifelian Daddyhole Limestone is succeeded by the Walls Hill Limestone and equivalent units representing the development of stromatoporoid reefs (Scrutton op. tit.). Apart from the coral faunas of Givetian aspect, the reef facies is poorly dated except in its upper part, and it is succeeded by the bioclastic Barton Limestone yielding terebratum Zone goniatites and varcus to Lr asymmetricus Zone conodonts (House 1963; Castle in Austin & Armstrong 1985). Inland, bedded back-reef limestones of broadly equivalent age occur, principally to the northwest as the Chercombe Bridge Limestone. To the west and north, the more or less continuous carbonate sequence thins, splits and passes fairly rapidly into deeper water argillites with limestone turbidites. These, the Nordon Slate, contain conodont faunas spanning the upper Eifelian to the Frasnian (Selwood & Durrance 1982; Selwood et al. 1984). Inland southwest of Tor Bay, the Ashprington Volcanic Group spilitic suite apparently continues through the Givetian but is only indirectly dated. Further north, the Eifelian Kingsteignton Volcanic Group, thought to be developed on the faulted northern margin of the South Devon Basin, acted as a separate locus for carbonate deposition of Givetian and early Frasnian age (Selwood et al. 1984; Selwood & Thomas 1986b). Further west on the outer shelf ridge around Plymouth, a similar pattern developed in another major carbonate complex (Orchard 1978) which was also initiated during the Eifelian. During the Givetian, back-reef facies were developed in the east, stromatoporoid reefs in the area of the city and deepwater argillites with limestone turbidites to the northwest. Both here and across to Tor Bay, palaeogeographical reconstruction is severely hampered by the structure, in which different successions are stacked up in a pile of northwardly displaced thrust sheets (Coward & McClay 1982; Selwood & Thomas 1986b). Such evidence as there is, however, suggests that the platforms may have had a considerable N-S extension. Here, as in Tor Bay+ volcanic activity may have influenced development of the carbonate platform. In eastern Cornwall, shallow water argillaceous facies persisted from the Eifelian through the Givetian; they yield rich Eifelian trilobite faunas, but Givetian dating is indirect (Burton & Tanner 1986). On the north Cornish coast, the Trevose Group is a continuation of the monotonous basinal argillites from the Eifelian (House et al. 1977; Beese 1982). In the dark grey slates, thin pale grey distal turbidites are common, as are bedding planes crowded with So'liolina. Goniatites are fairly widespread and although the molarium Zone itself is not recorded, a probable early Givetian fauna is represented and terebratum Zone faunas are widespread (House 1963; Gauss & House 1972). In south Cornwall, Holder & Leveridge (1986) identified two contemporary, dominantly clastic sequences northwest and southeast of the "Carrick Thrust'. To the northwest the Meadfoot Slate passes up transitionally into the dark grey and grey green slates with interbedded greywackes of the Gramscatho Group. The latter, although sparsely dated, probably embraces at least parts of both the Eifelian and Givetian as well as extending up into the Upper Devonian. Volcanic rocks of an oceanic tholeiitic suite (Floyd 1984) are probably of Eifelian age: they are interpreted as evidence of generation of oceanic crust, at least locally, within the Gramscatho Basin (Badham 1982: Barnes 19.83), regarded as initiated in the late Early Devonian by Barnes & Andrews (I 986). Southeast of the Carrick Thrust, the well dated Eifelian calciturbidites of presumed southern derivation (Sadler 1973) are succeeded by essentially intrabasinal (the Carne Formation) m+langes of Givetian and Frasnian age (Holder & Leveridge 1986). The two formations together comprise the Meneage Formation of Barnes (1983) and Barnes &
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
62
DEVONIAN
Andrews (1986). Barnes (1983) has described a gradational contact between the top of the calciturbidites through a few metres of mudstone with black chert into the base of the mrlange. Phacoids in the mrlange have produced sparse Ordovician faunas from quartzites (see O5b) and Devonian conodont faunas of Lochkovian to Givetian age from limestones. There is no detritus recognizable as directly derived from the Lizard ophiolite suite, although a southerly source for the mrlange is suggested (Barnes 1983, 1984). However, autochthonous and allochthonous volcanics have MORB affinities (Barnes & Andrews 1986). In north Devon the marine Lynton Slate (q.v.) was succeeded by the southward progradation of the alluvial plain facies of the Hangman Group in mid Eifelian times. Details of the facies are to be found in Tunbridge & Whittaker in Scrutton (1978). Much of the sequence was sourced in South Wales, but the rudaceous Rawns Formation has a distinctive clast assemblage suggesting derivation from Precambrian and Lower Palaeozoic rocks of an intermittently elevated landmass in the Bristol Channel (Tunbridge 1986) interpreted by Cope & Bassett (1987) as the local rejuvenation of the northern margin of the Pretannian block. In the earliest Givetian, a northward transgression returned marine conditions to the area. The Ilfracombe Slate is a sequence of argillaceous sediments with thin to thick beds and lenses of siltstones, sandstones and bioclastic limestones developed at various levels, consistent with an overall steady deepening of the environment through the Givetian (Edmonds et al. 1985; Edmonds & Williams 1985). Internal dating is poor; a single conodont record of upper varcus Zone age is associated with the uppermost slates of the group (Orchard 1979), whilst the coral faunas of the main carbonate horizons are of Givetian aspect (Scrutton 1975). The detailed palaeogeography of the area has been described by Webby (1966). Over the whole of the Wales/Welsh Borderland area, save south Pembrokeshire (see below), there are no deposits of known Middle Devonian age. The progressive increase in coarse detritus during the Pragian and Emsian, during which clasts and heavy mineral suites of semi-local derivation became predominant over the further-travelled alluvial detritus, is a reflection of the progressive uplift of the Welsh Basin and surrounding areas. The final phase of the Caledonian orogenic movements, the Acadian phase, took place during Middle Devonian times and was of major significance in the development of Caledonian structures and cleavage over much of the region. The major river systems draining the Greenland and Scandinavian Caledonides were clearly diverted away from Britain at this time, and sedimentation, which had been barely interrupted since early Cambrian times, was finally brought to a halt. In south Pembrokeshire, the Ridgeway Conglomerate (Williams 1971) contains phyllite and sedimentary clasts yielding fossils with a possible agerange of mid Cambrian to mid Ordovician, clearly derived from the south. Cope & Bassett (1987) have re-interpreted the conglomerate as having been derived from a rejuvenated northern margin of Pretannia, just as the Rawns Formation was, possibly slightly earlier (q.v.). There is no good age constraint on the age of the Ridgeway Conglomerate, since both its lower and upper boundaries are unconformities. In southeast England, the Bushey Borehole yielded fragmentary fossils including crinoids, brachiopods and gastropods, none of which was generically identifiable, together with fish and Tentaculites. The fossils were contained in variegated sands and clays with limestone. The description is here interpreted as implying a littoral zone. Eastwards from there, the Harmansole Borehole showed mottled sandstones, conglomerates and siltstones exhibiting rippling and rain pitting. These yielded estheriids, fish, plant remains, spores and acritarchs, suggesting a shoreline position indicated on the map. The Tatsfield Borehole yielded Thamnopora from limestone, suggesting a position farther offshore and this is confirmed by the varied marine fauna from the Bolney No. 1 Borehole, which, though not as extensive as the fauna from the Boulonnais (Ager & Wallace 1966) suggests open water. (See House et al. 1977 for further references.) One of the early surprises from the North Sea exploration was the discovery of a thick marine Middle Devonian sequence in the region of the Argyll Field. The succession penetrated in Well 30/24-3 included 93 m of marine Middle Devonian (base not reached). In the lower part of this was a thick limestone succession, of which 34m was proved, containing rugose and tabulate corals (Pennington 1975). Palaeogeographically the presence of a thick marine Middle Devonian sequence here is anomalous, and requires the presence of a narrow gulf extending northwards as depicted on the map. Central Scotland was uplifted along with the rest of the central part of the British Isles in the Middle Devonian and consequently there are no Middle Devonian rocks recorded from the Midland Valley. Rocks which pass up into the Carboniferous and are thought to be Upper Devonian rest with angular discordance on Lower Devonian rocks, the youngest of which occur in the axis of the Strathmore Syncline and are Emsian in age. Although the unconformity separating the Lower from the Upper Devonian is on the south limb of the syncline, there can be no doubt that this fold was generated during Middle Devonian times; it has a steep, locally overturned northwest limb and a complementary anticlinal structure in the Sidlaw Hills. The axis of the fold is parallel with the Highland Boundary Fault belt, which in places dips as low as 60 ~ to the northwest. It is clear that a reverse movement on the fault brought the Dalradian closer to the Midland Valley and generated the fold. In the south of the Midland Valley there is again an unconformity between Upper and Lower Old Red Sandstone, but the nature of the preUpper Devonian structures is not easily defined. It is likely that folding was
accompanied by substantial faulting; the impersistent folds have axes roughly parallel with the Southern Uplands Fault, and the Southern Uplands may have been translated to the northwest along it as envisaged by Kennedy (1958). The substantial changes in palaeogeography between the Lower and Middle Devonian resulted in sedimentation in Scotland being confined to areas north of the Highland Boundary Fault. Igneous activity had, by now, almost ceased in mainland Scotland. Middle Devonian rocks occur in three distinctive fault blocks on the Isles of Shetlands; on two of those, the eastern and the western, the rocks are of Givetian age. In the west, the Melby Formation, partly of Eifelian age, comprises fluvial sandstones which interfinger with lacustrine sediments which are overlain by andesitic and rhyolitic rocks. In the east are sandstones and conglomerates with some fish-bearing lacustrine rocks. On the Orkney Islands, dominantly east-flowing streams drained into a lake with lakeshore sediments and aeolian dunes; these were subsequently covered by the basaltic Hoy volcanics. Caithness has a thick lacustrine sequence which begins in the Eifelian and ends in the early Givetian with Givetian rocks overlapping Eifelian to rest directly on basement. These cyclical lacustrine sediments record the interplay between lake-level change and sediment supply (Trewin 1986). They have yielded a rich and important fish fauna which includes acanthodians, placoderms, crossopterygians, lungfish and palaeoniscids. Between the Caithness Flags and the Moine Thrust are a number of small basins which probably developed during the early Devonian; their sediments were then unconformably overlain by Middle Devonian rocks (Blackbourn 1981). These basins formed in the midst of ground which had either undergone, or was undergoing, cooling; their nature and significance has yet to be determined. Sandstones and flagstones, fish-bearing at the base, occur around the Moray Firth. The sandstones grade westwards into conglomerates. Middle Old Red Sandstone crops out around the Moray-Banff coast where it rests unconformably on Lower Old Red Sandstone. The sequences often begin with breccias which are replaced upwards by sandstones which often contain calcretes. Conglomerates and breccias of believed Middle Devonian age occur on either side of the Great Glen Fault; they contain clasts of local derivation. Along the projection of the Midland Valley into Ireland the same unconformable relationships exist. Upper Old Red Sandstone rests on Lower, but there is far less age control. Rocks of Middle Devonian (Eifelian) age are found in Clew Bay although it is uncertain whether they occur to the north or south of the Highland Boundary Fault (Graham et al. 1983). BJB, JCWC, CTS
D4: Frasnian-Early Famennian In South Devon the carbonate platform persisted into Frasnian times. In Torquay the highest parts of the Barton Limestone are of Frasnian age and lunulicosta Zone goniatites have been recorded from the succeeding slates. Lr a s y m m e t r i c u s Zone limestones also occur in southern Tor Bay with tufts of equivalent age to their north (Drummond 1982). These are related to the Ashprington Volcanic Group whose main centre was presumably still active inland although not dated at this level. Northwest of Tor Bay, carbonate deposition persisted almost to the end of the Frasnian, with coralrich, pink, micritic bioherms yielding good conodont faunas (Scrutton 1978; Selwood et al. 1984). Elsewhere, argillites largely referred to the Nordon Slate, with a few distal turbidites, became more widespread and pass transitionally upwards into purple and green Gurrington Slate in the South Devon Basin. On the outer shelf rise to the south, the laterally equivalent Luxton Nodular Limestone developed over the carbonate platform later in the Frasnian. By late Frasnian times basinal facies had spread throughout the area. Although continuing subsidence is implied, successions on the rise (schwelle) are still thinner than the ostracod-bearing Gurrington Slate of the deep basinal area (Selwood & Durrance 1982; Selwood et al. 1984). Further west on the Plymouth area rise, carbonate deposition also persisted well into the Frasnian, and locally, as indicated by conodonts, into the Famennian (Orchard in House et al. 1977). There was erosion and crevassing of the former reef core in the late Frasnian. Northwest of the carbonates, a basinal ostracod slate facies was established early in the Frasnian, although dating is not precise (Gooday 1975). Throughout South Devon these successions are stacked up in a pile of northwardly displaced thrust sheets (Isaac et al. 1982; Selwood et al. 1984; Selwood & Thomas 1986b). Around Chudleigh, a local rise yields rich faunas of goniatites from nodules in a highly condensed sequence ranging through the Frasnian and up into the Dinantian (House & Butcher 1973). Between Dartmoor and Bodmin Moor, Isaac et al. (1982) have demonstrated a range of laterally contemporaneous Famennian basin and rise facies dated by conodonts, clymeniids, trilobites and ostracods. A rise environment is envisaged for the clastic and carbonate (nodular cephalopod limestone or lenticular bioclastic bivalve/brachiopod limestone) facies; basinal facies are monotonous grey. green or purple and green argillites. The latter yield a pelagic Frasnian fauna in east Cornwall (Burton & Tanner 1986). Further west, on the north Cornish coast, a varied Frasnian sequence includes the Marble Cliff Limestone----caiciturbidites of probable southern derivation--which have yielded excellent conodont faunas ranging up into the lower part of the Frasnian (House, Mouravieff & Beese in Scrutton
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
64
DEVONIAN
1978). They are succeeded by an argillaceous sequence containing thin tufts and agglomerates; these volcaniclastics are undated, but underlie green argillites with purple bands dated as mid Frasnian by conodonts and goniatites (Beese 1984). A contrasting facies is closely juxtaposed to the north of the Camel Estuary--including the thick Pentire Pillow Lavas (of inferred Frasnian age). The later Frasnian, together with the early and possibly mid Famennian is contained within the purple and green ostracodbearing Polzeath Slate (Beese 1984). In south Cornwall, grey and black argillites with thin sandstones, accompanied by basic volcanics, comprise the Mylor Slate which succeeds the Gramscatho Group northwest of the 'Carrick Thrust'. Recent palynological evidence (Wilkinson & Knight 1989) suggests that the facies change was late Famennian, much later than previously supposed. This is consistent with earlier palynomorph results from the Portscatho Formation (Le Gall et al. 1985) and the Mylor Slates (Turner et al. 1979). To accommodate the new evidence, Wilkinson & Knight (1989) suggest that the Carrick Thrust is actually a relatively minor structure within a horst bounded by the main Blue Rock Thrust 1 km to the south and the sole thrust at Caca-stull Zawn. In the allochthon southwest of this zone, extension of the m61ange into the Frasnian is indicated by a conodont record from limestones associated with pillow lavas on Mullion Island (Hendriks et al. 1971). Barnes & Andrews (1986) regard the Lizard ophiolite as of late Givetian-early Frasnian origin, in an intracratonic leaky transform setting and emplaced cold during Carboniferous thrusting. Holder & Leveridge (1986) in contrast, envisage an earlier origin (c. 400 Ma) in a more extensive, if narrow, ocean. They suggest progressive northward thrust emplacement carrying the Lizard ophiolite into its present position; erosion on the front of the thick nappe pile provided debris for Gramscatho lithologies as grains, clasts and blocks in the upper 500 m of the Mylor Slate. In north Devon, sedimentation is assumed to have continued more or less uninterrupted from the Givetian through to the end of the Devonian, but the sequence is very poorly dated. The Ilfracombe Slate is succeeded by the Morte Slate which has yielded only a few poorly preserved bivalves and brachiopods and for which a Frasnian and possibly early Famennian age is accepted (Selwood & Durrance 1982; Edmonds et al. 1985). The sequence is predominantly slates with thin sandstones which have been interpreted by Webby (1966) as a part of a prodelta~lelta platform complex. The Morte Slate is succeeded by the thick, red, purple and green sandstones, siltstones and mudrocks of the fluviatile and lacustrine Pickwell Down Sandstone (Selwood & Durrance 1982) marking the culmination of the second major regressive episode in the Devonian rocks of the area. Some fish remains occur, but no precise dates are available and the unit is regarded as early Famennian on evidence of succession. The Upper Old Red Sandstone of the Anglo-Welsh area is known collectively as the Farlow Group. The succession is generally thin in comparison with the Lower Old Red Sandstone and dating, largely by fish, is imprecise. A general equivalence with the late Frasnian and Famennian seems probable and in many areas the earliest Carboniferous rocks are in Old Red Sandstone facies (see Map C1). In the Brecon Beacons, the Plateau Beds are pink and red quartzitic sandstones with conglomeratic courses and mudstones yielding a fauna of fish, together with Cyrtospirifer verneuili, Ptychomalotoechia omaliusi and other marine fossils, indicating a periodic marine influence well to the north at this time and a probable late Frasnian or early Famennian age (see House et al. 1977 for earlier refs.). The overlying Grey Grits are quartzites and quartz pebble conglomerates--a facies which is widespread at this horizon in South Wales. In the Bristol area the Portishead Beds (c. 500 m max.) overlap the Lower Old Red Sandstone locally and rest on the Silurian (Kellaway & Welch 1955). In southeast England the Apley Barn Borehole yielded Hypothyridina cf. cuboides and other marine fossils, some of which were reworked, suggesting a position fairly close to the shoreline. A position right on the shoreline is indicated by the Wyboston Borehole in which large plant fragments occur in grey and purple mudstones and siltstones with a sparse fauna of chitinozoa and scolecodonts together with acritarchs. A wide littoral zone is implied by three boreholes to the south of this point, all in the London area. The Tottenham Court Road, Stonebridge Park and Willesden No. I boreholes all revealed Old Red Sandstone facies associated with marine faunas (see House et al. 1977, for earlier refs.). In the Midlands the Merevale No. 2 Borehole (Taylor & Rushton 1971) yielded marine fossils including lingulids, bivalves and rhynchonellids from red sandstones. Much of the upper part of the Upper Old Red Sandstone of the Midland Valley of Scotland is of Carboniferous age; there is an imperceptible passage upwards from the Devonian, and it has only been the advent of palynomorph biostratigraphy which has enabled these rocks to be dated. Thus some of the Old Red Sandstone is described under the Carboniferous (q.v.). In the region of the Firth of Clyde there are fairly extensive developments of boulder-bearing conglomerates, developed near contemporary fault scarps. These grade up into finer conglomerates and then to sandstones and mudstones with calcretes. This upward-fining megasequence oversteps underlying rocks in a southerly direction, where calcretes become more abundant and thicker, on what was a mature surface with small streams between widely developed calcretes. No such overstep occurs on the NW margin of the basin, except in Kintyre where, on the southern tip of the peninsula, conglomerates and sandstones are replaced by sandstones with calcretes resting on the Dalradian south of Machrihanish.
In the region north of Saltcoats alluvial sandstones within this megasequence have channel and floodplain/floodbasin deposits suggesting deposition from a deep river system; the largest bar recorded is 8 m thick, implying deposition from a river system > 10m deep (Bluck 1981). It is difficult to envisage the local uplands supporting a river system of this magnitude, and speculatively a river from the north is suggested. This river, flowing eastwards was probably diverted by the Southern Uplands high along the Midland Valley and out into the North Sea where a deltaic facies is found. The Upper Old Red Sandstone of the Firth of Clyde Basin may be 3 km thick; the basin probably owes its origin to strike-slip movement along a system of faults related to the Highland Boundary Fault (Bluck 1980) along which it is largely terminated. The nature of the. source blocks and the contributions of the Dalradian block are unknown but latest Devonianearly Carboniferous rocks along the Highland Boundary Fault belt show a clear Dalradian contribution. The seabed west of Shetland exposes sandstones and pebbly sandstones of Devonian-Carboniferous age which consist of stacked channel deposits of braided and meandering streams (Meadows et al. 1987). Sequences on the Isles of Shetland and Orkney and the north Scottish mainland are typically sandstones laid down by shallow braided rivers, bordered by floodplains, some of which bore aeolian dunes. A rare example of Upper Devonian basalts occurs in the Orkneys. In the region between Nairn and Spey Bay the Upper Old Red Sandstone is diachronous from west-east and has thick calcretes. In Northern Ireland beds ascribed to the Upper Old Red Sandstone occur south of Cushendall. These, as with the conglomeratic beds of Kintyre, probably have a local origin (Simon 1984b). The southern half of Ireland has an extensive development of Upper Old Red Sandstone. Recent miospore biostratigraphy has confirmed the longheld suspicion that the Old Red Sandstone facies persisted into Carboniferous times (q.v.). The fullest development of the Upper Old Red Sandstone is to be seen in the Munster Basin, a depositional area lying southwest of an approximate line from Limerick to Waterford. Graham (1983) has summarized details of the basin form, isopachytes and palaeoflows and recognized four main facies. The greatest thickness is in the Comeragh Mountains, southwest of Killarney, where the group exceeds 6000 m. The succession begins with a marginal alluvial fan facies, consisting in the north and east of over 300 m of coarse cobble conglomerates, including clasts of various igneous rocks, greywacke, slate and quartzite; matches can be made with the Leinster granite and its aureole. These conglomerates are succeeded by quartz conglomerates and sandstones deposited basin-wide by a series of braided streams. A fine-grained alluvial facies, which forms the bulk of the sediment, may have been the accretions of the alluvial channels and floodplains. Finally there is a fluvial coastal plain facies which includes the deposits of floodplains, lev6es, crevasse splays and which yields important floras. Palaeocurrents were variable, but dominantly to the south, although a westerly flow is indicated for the eastern margin of the basin. Where the base is seen, around the margins of the basin, there is a sharp unconformable contact with the Lower Palaeozoic. The succession clearly post-dates the mid Devonian deformation and its age ranges from within the Frasnian up into the Carboniferous. Price & Todd (1988) have expressed the view that the Munster Basin opened on pre-existing fractures, and the same (or similar) fractures were utilized during its closure. BJB, JCWC, CTS R
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References not listed here are cited in House et al. (1977). AGER, D. V. & WALLACE,P. 1966. The environment history of the Boulonnais, France. Proceedings of the Geologists'Association, 77, 385-417. ALLEN,J. R. L. 1983. Studies in fluviatile sedimentation: bars, bar-complexes, and sheet sandstones (low sinuosity streams) in the Brownstones (L. Devonian), Welsh Borders. Sedimentary Geology, 33, 237-293. 1985. Marine to freshwater: the sedimentologyof the interrupted environmental transition (Ludlow-Siegenian) in the Anglo-Welsharea. In: CnALONER,W. G. & LAWSON,J. D. (eds) Evolution and environment in the late Silurian and early Devonian. Philosophical Transactions of the Royal Society, B309, 85-104. & WILUAMS,B. P. 1978. The sequence of the earlier Lower Old Red Sandstone (Siluro-Devonian), north of Milford Haven, south-west Dyfed (Wales). Geological Journal, 13, 113-136. & -1981. Sedimentology and stratigraphy of the Townsend Tuff Bed (Lower Old Red Sandstone) in South Wales and the Welsh Borders. Journal of the Geological Society, London, 138, 15-29. & - - 1982. The architecture of an alluvial suite: rocks between the Townsend Tuff and Pickard Bay Tuff Beds (early Devonian), southwest Wales. Philosophical Transactions of the Royal Society, B297, 51-89, pls. I-5. AUSTIN, R. L. & ARMSTRONG,H. A. (eds) 1985. Fourth European Conodont Symposium (Ecos IV), Field Excursion A. BADHAM, J. P. N. 1982. Strike-slip orogens--an explanation for the Hercynides. Journal of the Geological Society, London, 139, 493-504. BARNES, R. P. 1983. The stratigraphy of a sedimentary melange and associated deposits in South Cornwall. Proceedings of the Geologists' Association, 94, 217229. 1984. Possible Lizard-derived material in the underlying Meneage Formation. Journal o[ the Geological Society, London, 141, 79-85. - - & ANDREWS,J. R. 1986. Upper Palaeozic ophiolite generation and obduction in south Cornwall. Journal of the Geological Socieo', London, 143, 117-124. BEESE,A. P. 1982. The argillite facies of the Middle Devonian successions in north Cornwall. Proceedings of the Ussher Society, 5, 321-332. -
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1984. Stratigraphy of the Upper Devonian argillite succession in north Cornwall. Geological Magazine, 121, 61-69. BLACKBOURN,G. A. 1981. Correlation of Old Red Sandstone (Devonian) outliers in the Northern Highlands of Scotland. Geological Magazine, 118, 409-414. BLUCK, B. J. 1980. Evolution of a strike-slip, fault-controlled basin, Upper Old Red Sandstone, Scotland. In: BALLANCE,P. F. & READING,H. G. (eds) Sedimentation in Oblique-slip Mobile Zones. International Association of Sedimentologists, Special Publication, 4, 63-78. -1981. Upper Old Red Sandstone, Firth of Clyde. In." ELLIOTT,T. (ed.) Field Guides to Modern and Ancient Fluvial Systems in Britain and Spain. International Fluvial Conference, University of Keele, 5.14-5.20. -1984. Pre-Carboniferous history of the Midland Valley of Scotland. Transactions of the Royal Society of Edinburgh." Earth Sciences, 75, 275-295. 1990. Terrane provenance and amalgamation: some examples from the Caledonides. Philosophical Transactions of the Royal Society, A331, 599-609. BURTON,C. J. & TANNER,P. W. G. 1986. The stratigraphy and structure of Devonian rocks around Liskeard, east Cornwall, with regional implications. Journal of the Geological Society, London, 143, 95-105. COPE, J. C. W. 1981. The Swansea Valley Fault, Wales. Geological Magazine, 118, 309-310. 1988. The pre-Devonian geology of South-west England. Proceedings of the Ussher Society, 7, 468-473. & BASSETT,M. G. 1987. Sediment sources and Palaeozoic history of the Bristol Channel area. Proceedings of the Geologists' Association, 98, 315-330. COWARD,M. P. & MCCLAY,K. R. 1982. Thrust tectonics of S. Devon. Journal of the Geological Society, London, 140, 215-228. DEC, T. 1988. Development of the Lower and Middle Old Red Sandstone sedimentary Basin in Eastern Sutherland, North-West Scotland, PhD thesis, University of Glasgow. DONOVAN,R. N. 1975. Devonian lacustrine limestones at the margin of the Orcadian basin, Scotland. Journal of the Geological Society, London, 131,489-510. 1980. Lacustrine cycles, fish ecology and stratigraphic zonation in the Middle Devonian of Caithness. Scottish Journal of Geology, 16, 35-50. DRUMMOND, M. E. 1982. The Geology of the Devonian Limestones of the BrixhamDartington area, South Devon. PhD thesis, University of Newcastle upon Tyne. EDMONDS, E. A., WHITTAKER,A. & WILLIAMS,B. J. 1985. Geology of the country around Ilfracombe and Barnstaple. Memoir of the Geological Survey of Great -
-
-
-
-
-
-
-
-
-
Britain. & WILLIAMS, B. J. 1985. Geology of the country around Taunton and the Quantock Hills. Memoir of the Geological Survey of Great Britain. FLOYD,P. A. 1984. Geochemical characteristics and comparison of the basic rocks of the Lizard Complex and the basaltic lavas within the Hercynian troughs of SW England. Journal of the Geological Society, London, 141, 61-70. & LEVERIDGE,B. A. 1986. Lithological and geochemical characteristics of some Gramscatho turbidites, south Cornwall (Abstract). Proceedings of the Ussher Society, 6, 421. GOLDR1NG, R. & LANGENSTRASSEN,F. 1979. Open shelf and near-shore clastic facies in the Devonian. In: HOUSE,M. R., SCRUTTON,C. T. & BASSETT,i . G. (eds) The Devonian System. Special Papers in Palaeontology, 23, 81-97. GRAHAM,J. R. 1983. Analysis of the Upper Devonian Munster Basin, an example of a fluvial distributary system. In: COLLINSON,J. E. & LEWIN,J. (eds) Modern and ancient fluvial systems. International Association of Sedimentologists, Special Publication, 6, 473-483. , RICHARDSON,J. B. & CLAYTON,G. 1983. Age and significance of the Old Red Sandstone around Clew Bay, N. W. Ireland. Transactions of the Royal Society of Edinburgh, Earth Sciences, 73 (for 1982), 245-249. HAUGHTON,P. n . W. 1988. A cryptic Caledonian flysch terrane in Scotland. Journal of the Geological Society, London, 145, 685-703. & BLUCK, B. J. 1988. Diverse alluvial sequences from the Lower Old Red Sandstone of the Strathmore region, Scotland--implicationsfor the relationship between late Caledonian tectonics and sedimentation. In: McMILLAN, N. J., EMBRY, A. F. & GLASS,D. J. (eds) Devonian of the World. Canadian Society of Petroleum Geologists. HOLDER, M. T. & LEVERIDGE,B. E. 1986. A model for the tectonic evolution of south Cornwall. Journal of the Geological Society, London, 143, ! 25-134. HOLLAND, C. H. 1981. Devonian. In: HOLLAND,C. H. (ed.) A Geology oflreland Scottish Academic Press, Edinburgh, 121-146. HOUSE, M. R., RICHARDSON,J. B., CHALONER,W. G., ALLEN, J. R. L., HOLLAND, C. H. & WESTOLL, T. S. 1977. A Correlation of Devonian Rocks in the British Isles. Geological Society, London, Special Report, g. ISAAC, K. P., TURNER,P. J. & STEWART,I. J. 1982. The evolution of the Hercynides of central SW England. Journal of the Geological Society, London, 139, 521-523. KELLEY, S. 1988. The relationship between K-Ar mineral ages, mica grainsizes and movement on the Moine Thrust Zone, NW Highlands, Scotland. Journal of the Geological Society, London, 145, 1-10. KENNEDY, W. Q. 1958. The tectonic evolution of the Midland Valley of Scotland. Transactions of the Geological Society of Glasgow, 23, 106-133. KIMBER, R. 1984. Clastic deposition beneath the lowermost Carboniferous Limestones of north-west England. European Dinantian Environments 1st meeting, Manchester, Abstracts. Department of Earth Sciences, Open University. LE GALL, B., LE Hf~RISSLA. & DEUNFF,J. 1985. New palynological data from the Gramscatho Group at the Lizard front (Cornwall); palaeogeographical and geodynamical implications. Proceedings of the Geologists' Association, 96, 237 253.
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-
-
MATTHEWS,S. C. 1977. The Variscan foldbelt in southwest England. Neues Jahrbuch fiir Geologic und Paliiontologie, Abhandlungen, 154, 94-127. MCLAREN, D. J. 1977. The Silurian-Devonian boundary committee. A final report. In MARTINSSON,A. (ed.) The Silurian-Devonian Boundary. International Union of Geological Sciences, A, 5, 1-34. MEADOWS, N. S., MACCHI, L., CUBITr, J. M. & JOHNSON, B. 1987. Sedimentology and reservoir potential in the west of Shetland, UK, exploration area. In: BROOKS, J. & GLENNIE, K. (eds) Petroleum Geology of North-West Europe. Graham & Trotman, London, 723-736. ORCHARD, M. J. 1978. The conodont biostratigraphy of the Devonian Plymouth Limestone, South Devon. Palaeontology, 21,907-955. -1979. On a varcus Zone conodont fauna from the Ilfracombe Slates (Devonian, north Devon). Geological Magazine, 116, 129-134. PRICE, C. A. & TODD, S. P. 1988. A model for the development of the Irish Variscides. Journal of the Geological Society, London, 145, 935-939. RICHARDS, P. C. 1985. A Lower Old Red Sandstone lake in the offshore Orcadian Basin. Scottish Journal of Geology, 21, 381-383. RICHARDSON,J. B., FORD, J. H. & PARKER,F. 1984. Miospores, correlation and age of some Scottish Lower Old Red Sandstone sediments from the Strathmore region (Fife and Angus). Journal of Micro-palaeontology, 3, 109-124. & MCGREGOR,D. C. 1986. Silurian and Devonian spore zones of the Old Red Sandstone continent and adjacent regions. Bulletin of the Geological Survey of Canada, 364, 1-79. SCRUTTON, C. T. 1977a. Reef facies in the Devonian limestones of eastern South Devon, England. Mdmoires du Bureau de Recherches Gdologiques et Minidres, 125-135. 1977b. Facies variations in the Devonian limestones of eastern South Devon. Geological Magazine, 114, 165-193. - - (ed.) 1978. A field guide to selected areas of the Devonian of south-west England. The Palaeontological Association, London. SELWOOD, E. B. & DURRANCE, E. M. 1982. The Devonian rocks. In: DURRANCE, E. M. & LAMING,D. J. C. (eds) The geology of Devon, University of Exeter, 1541. -EDWARDS, R. A, SIMPSON, S., CHESHER, J. A., HAMBLIN, R. J. O., HENSON, M. R., RIDDOLLS,B. W. & WATERS,R. S. 1984. Geology of the country around Newton Abbot. Memoir of the Geological Survey of Great Britain. ,s THOMAS,J. M. 1986a. Upper Palaeozoic successions and nappe structures in north Cornwall. Journal of the Geological Society, London, 143, 75-82. & -1986b. Variscan facies and structure in central SW England. Journal of the Geological Society, London, 143, 199-207. SHACKLETON, R. i . , RIES, A. C. & COWARD,M. P. 1982. An interpretation of the Variscan structures of SW England. Journal of the Geological Society, London, 139, 533-541. SIMON, J. B. 1984a. Provenance and depositional history of the Lower Old Red Sandstone of northeast Antrim. Irish Journal of Earth Sciences, 6, 1-13. 1984b. Sedimentation of a small complex alluvial fan of possible Upper Old Red Sandstone age, northeast County Antrim. Irish Journal of Earth Sciences, 6, 109-119. 1984c. Sedimentation and tectonic setting of the Lower Old Red Sandstone of the Fintona and Curlew mountain district. Irish Journal of Earth Sciences, 6, 213-228. STEWART,I. J. 1981. Late Devonian and Lower Carboniferous conodonts from north Cornwall and their stratigraphic significance. Proceedings of the Ussher SocieO', 5, 179-185. TAYLOR, K. & RUSHTON, A. W. A. 1971. The pre-Westphalian geology of the Warwickshire Coalfield. Bulletin of the Geological Survey of Great Britain, THIRLWALL,i . F. 1988. Geochronology of Late-Caledonian magmatism in northern Britain. Journal of the Geological Society, London, 145, 951-961. THOMAS, R. G. 1978. The Stratigraphy, Palynology and Sedimentology of the Lower Old Red Sandstone Cosheston Group, S. W. Dyfed, Wales. PhD thesis, University of Bristol. TREWIN,N. H. 1986. Palaeoecology, and sedimentology of the Achanarras Fish Bed of the Middle Old Red Sandstone, Scotland. Transactions of the Royal Society of Edinburgh, Earth Sciences, 77, 21-46. TUNBRIDGE,I. P. 1980. Possible Devonian uplift on the Swansea Valley Fault, Wales. Geological Magazine, 117, 497-498. 1981. Old Red Sandstone sedimentation--an example from the Brownstones (highest Lower Old Red Sandstone) of south central Wales. Geological Journal, 16, 111-124. 1986. Mid-Devonian tectonics and sedimentation in the Bristol Channel area. Journal of the Geological Society, London, 143, 107-115. TURNER, P. J. 1986. Stratigraphical and structural variations in central SW England: a critical appraisal. Proceedings of the Geologists' Association, 97, 331-345. TURNER, R. E., TAYLOR, R. T., GOODE, A. J. J. & OWENS, B. 1979. Palynological evidence for the age of the Mylor Slates, Mount Wellington, Cornwall. Proceedings of the Ussher Society, 4, 274-283. WILKINSON, J. J. & KNIGHT, R. R. W. 1989. Palynological evidence from the Porthleven area, South Cornwall: implications for Devonian stratigraphy and Hercynian structural evolution. Journal of the Geological Socieo', London, 146, 739-742. -
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Carboniferous J . C. W. C O P E ,
P. D. G U I O N ,
G. D. S E V A S T O P U L O
Carboniferous geology has developed very rapidly over the past two decades since ideas on sedimentology and new radiometric scales, together with new biostratigraphical schemes based on miospores, conodonts and foraminifera have been synthesized with ideas on plate tectonic processes, climatic changes and oscillations of sea level. The application of the McKenzie model of basin development to the Carboniferous, and the relating of eustatic sea-level changes, which had a profound effect on Carboniferous palaeogeography, to fluctuations in the size of the Gondwanan ice sheets, have been major factors in the understanding of Carboniferous processes. The Carboniferous evolution of Britain took place to the (present) north of a plate suturing occurring along a line extending from Galicia, through Armorica and the Massif Central to the Vosges, during mid Devonian to early Carboniferous times. In front of this collision zone was the RhenoHercynian zone which opened probably during mid Devonian times and which was partly floored by oceanic crust; its closure can be traced through to late Carboniferous times as a series of northwardly directed thrust complexes. To the (present) north of this zone was a foreland on which basins developed on either side of a persistent Wales-London-Brabant High. Studies by Dewey (1982) and Leeder (1982, 1987, 1988) have shown that late Devonian to early Carboniferous extension, involving 13 factors of up to 2, produced a series of grabens and half-grabens in the Caledonian basement. These developing structures were the dominant control on early Carboniferous sedimentation and produced a sea-bed topography which was reinforced by patterns of sedimentation; the basins often receiving little sediment, whilst on the adjacent platforms major build-ups of carbonate sediment occurred. On the intervening horst blocks sedimentation was frequently condensed. Important climatic changes occurred from an arid climate, in earliest Carboniferous times to semi-arid and then monsoonal in later Dinantian times (Besly 1987). Related to this increasing rainfall may have been the southward progradation of deltaic sediments, carried by a river of Mississippi proportions, which had a profound influence on sedimentation over areas to the north of the Wales-London-Brabant High, Sedimentation in the Namurian was initially affected by sea-bed topography inherited from the Dinantian, but as crustal extension gave way to complementary thermal subsidence, thick, more uniform sequences of deltaic sediments accumulated in many areas of northern Britain through to about mid-Westphalian B times. Around this latter time thermal subsidence was halted by the progressive 'northward' movement of the Variscan deformation front. Depositional regimes in later Westphalian times were largely dictated by fault patterns and this remained the dominant feature through to the main Variscan deformation in Autunian times. Subsystems. At the time of publication of the Geological Society's Correlation Charts on the Carboniferous, the period was divided into Dinantian (George et al. 1976) and Silesian Subsystems (Ramsbottom et al. 1978). A recent decision by the Carboniferous Subcommission, incorporated onto the I.U.G.S. Global Stratotype Chart (1989) has led to a new division of the Carboniferous into two subsystems, the boundary being drawn within the early Chokierian Stage at a proposed British stratotype in north Yorkshire (Riley et al. 1987). This boundary produces new subsystems, as yet unnamed, which are equivalent neither to the Dinantian/Silesian nor the Mississipian/Pennsylvanian. In this atlas Dinantian is used as an informal term in the descriptions accompanying the maps. Series. The I.U.G.S. 1989 Global Stratotype Chart recognizes in the lower subsystem the Tournaisian, Vis6an and part of the Namurian Series. In the upper subsystem the remainder of the Namurian, the Westphalian and Stephanian Series are recognized. Stages. The British stages of the Carboniferous, as used in the Society's Correlation Charts, are followed here. There are serious reservations about the suitability of several of the Dinantian stages, which closely coincide with the mesothemic boundaries of Ramsbottom (1973). Since that time it has become clear that it is difficult to define some of the stage boundaries away from their stratotypes, since the boundaries are not always drawn at biostratigraphically recognizable points. There are moves to redefine some of these stage boundaries in line with biostratigraphical reference points. On the Carboniferous maps we have attempted no palinspastic reconstruction for southwest England because of the complex nature of the sequential thrust stacking which has occurred in Cornwall. In considering our palaeogeographical reconstructions therefore, note should be made of the considerable crustal shortening in southwest England. JCWC 9 1992The GeologicalSociety
& A. R. H . S W A N
CI: Devonian-Carboniferous boundary (latest Famennianearliest Courceyan) The Devonian-Carboniferous boundary in the British area has traditionally been drawn at the facies change between the fluviatile sediments of the Old Red Sandstone and the marine limestone and shale sequence of the Lower Carboniferous. At this point, biostratigraphical data are scanty over much of Britain and the usefulness of Vaughan's K Zone (with its index fossil Cleistopora( = Vaughania) vetus) has been questioned. In the marine successions of southwest England the boundary between the Devonian and Carboniferous can be identified accurately in those few localities which have yielded ammonoids. In the Launceston area of Cornwall, Selwood (1960) recorded Wocklumeria, characteristic of the highest Famennian (Stourscombe Beds) overlain by the Yeolmbridge Beds with earliest Carboniferous Gattendorfia. Farther to the east, in the Chudleigh area, shales with nodules have yielded rich faunas of ammonoids, trilobites and ostracods (House et al., 1977) and are succeeded by early Carboniferous cherts with conodonts. In north Devon the Pilton Beds yield latest Famennian faunas in their lower divisions and pass imperceptibly up into the Carboniferous in their upper part (Goldring 1955, 1970; House et al. 1977; Edmonds et al. 1985). The Knap Farm Borehole at Cannington Park, Somerset (Whittaker & Scrivener 1982) marks the southwestern limit of the widespread shallow shelf limestone and shale facies (the Lower Limestone Shales) of the southwestern Province. An earliest Carboniferous age is now clearly established at both Knap Farm and at Burrington Combe on the basis of miospores (Higgs et al. 1988), but in the Avon Gorge, the type section for Vaughan's 'Avonian' Series (1905) the environment was not fully marine at this time and the succession is a facies intermediate between the Old Red Sandstone and the Lower Limestone Shales. The Taft Gorge section in South Wales (Gayer et al. 1973) provided one of the earliest sections in the southwest in which the pre-Carboniferous dating of the marine transgression was established. Earliest Carboniferous marine influence had long been known from southwest Dyfed (south Pembrokeshire) where the Skrinkle Sandstone (Dixon 1921) yields inshore marine faunas of conodonts and brachiopods, together with spores of a VI Biozone flora, the latter appearing in the top 3 m of the Skrinkle Sandstone at West Angle Bay (Dolby 1970). In southeast England, lowest Carboniferous limestones are recorded in boreholes at Harmansole (Smart et al. 1966) and at Warlingham (Worssam & lvimey-Cook 1971). To the north of this area, boreholes at Cambridge and at Gayton (Worssam & Taylor 1969) both recorded basal Carboniferous in non-marine facies which are here interpreted as lying on an alluvial plain to the north of the encroaching sea. Similar alluvial facies is probably represented in South Wales by the conglomeratic Grey Grits of the Brecon Beacons, whose age is likely to range from late Famennian to earliest Courceyan (House et al. 1977, fig. 7). In Ireland the first marine incursions into the southwest of the area were clearly of late Devonian age (Higgs et al. 1988). These latter authors have recently studied miospore assemblages across the Devonian--Carboniferous boundary of Ireland and have demonstrated biostratigraphically, what had long been suspected, but which could not be proven, namely that the marine transgression across Ireland was markedly diachronous. Thus Old Red Sandstone facies continued to accumulate in the Irish Midlands long after marine sedimentation had been fully established in the south. In the southwest, marine slates accumulated at the Devonian-Carboniferous boundary. The base of the VI Biozone was identified near, or at, the base of the Castle Slate Member of the Kmsale Formation on the north side of Dunmanus Bay, and from 0.1 m above the base of the Castle Slate Member at Old Head of Kinsale. The latter section is particularly important, as it is the boundary stratotype for the Courceyan Stage (George et al. 1976, p. 6). Here, the Old Head Sandstone Formation consists of tidal sands and muds, the upper part is more calcareous and with mud flasers. The Kinsale Formation consists of dark grey mudstones, which overlie the Old Castle Head Formation with an abrupt facies change. The Castle Slate Member is interpreted as offshore muds and contains lenses and thin beds of limestone. North of an approximate line from Cork to the Kenmare River (the Cork-Kenmare Line) the facies change from Old Red Sandstone to marine Carboniferous had not yet occurred, and fluvial Old Red Sandstone facies, becoming further removed from marine influence northwards, accumulated. 67
68
CARBONIFEROUS
Higgs et al. (1988--see also for earlier references) have shown that at Hook Head the VI Biozone occurs up to 16m below the top of the Old Red Sandstone and similarly occurs near the top of the Old Red Sandstone at Killarney and Kilworth--roughly along the depositional strike from Hook Head and Borehole WX 68.1 in the Duncormick-Wexford Outlier. A further record of the VI miospore biozone in Old Red Sandstone facies has been recorded by Higgs et aL (1988) from Kilmore. Farther to the north, at Slieve Aughty and Slieve Bloom the VI miospore biozone has not been identified hitherto, but in both localities the overlying HD miospore biozone is still in Old Red Sandstone facies (Higgs et al. 1988 pp. 35-6) demonstrating quite clearly that the equivalents of the VI Biozone must lie well within the Old Red Sandstone. The diachronous nature of this facies change is thus clearly established. The margins of the Upper Old Red Sandstone sedimentation are shown on the map to lie a short distance to the north of these latter localities, although the northward depositional limits are not clear. In Northern England red sandstones and conglomerates are recorded from boreholes at Duke's Wood and Caldon Low. In neither of these localities is the age of the formations tightly constrained as they are known to conformably underlie younger Carboniferous rocks, but in neither case has their maximum age been shown. The sediments in both localities appear to be of alluvial origin and have been interpreted as having accumulated in an alluvial basin environment to the north of the marine depositional area. Whether these two basins were connected is unknown. It is also unclear whether the drainage in the basin(s) was internal, or whether there was an alluvial outlet--and if the latter case--whether the outlet was to the south. Gawthorpe et al. (1989) have recently interpreted the Caldon Low sediments as the earliest deposits to accumulate in the fault-bounded north Staffordshire Basin (Lee 1988); as this is a structure elongated north-south, with fault boundaries to the east and west, it seems at least possile that drainage was to the south as indicated with a query on the map. It is also possible that the southern part of northern England may have contained several alluvial basins, some more of which may be discovered during the intensive hydrocarbon exploration activity in the area. Farther to the north, sedimentation of Old Red Sandstone facies sediments may have already begun in the Vale of Eden--a topographic low at this time. The northern end of the Vale of Eden may have been in sedimentological contiguity with an area of alluvial sedimentation which developed between the southeastern flank of the Southern Uplands High and the northeastern flank of the northern English land area, in an outcrop extending northeastwards to the coast around St Abb's Head. Various boreholes in the eastern North Sea have penetrated alluvial Old Red Sandstone facies, as indicated on the map. That of the Buchan Field, being the most northerly of the currently released North Sea wells in the region is particularly interesting as Old Red Sandstone facies persists through to the Vis6an (q.v.). Off northern Scotland, the Clair Basin (Meadows et al. 1987) contains a sequence of Devonian--Carboniferous sediments, the lower part of which is characterized by sandstones with some conglomerates. The dating of these beds is based on holoptychian fish scales, sparse palynology and radiometric dating (largely unpublished: see Ridd 1981). South of the Moray Firth, the upper part of the Old Red Sandstone outcrop of Morayshire and Nairnshire is of late Famennian or even early Courceyan age. The Rosebrae Beds apparently rest disconformably on earlier formations of the Upper Old Red Sandstone, which had undergone prior faulting; the Rosebrae Beds were recorded as lying on the Alves Beds to the west of the Rothes Fault, and the Cornstone Beds to the east of that fault (Peacock et al. 1968). The Rosebrae Beds contain a rich fauna of completely preserved fish, including the Famennian genus Phyllolepis, and an age at the top of the Famennian seems most appropriate. Mykura (in Craig 1983) placed the Rosebrae Beds in the upper part of the 'Phyllolepis Series' of Miles (1968). Westoll (in House et al. 1977) agreed that the Rosebrae Beds were in the upper part of the Famennian, but placed their top just below the 'Strunian' (= Tn la of the Belgian notation). Rogers et al. (1989) suggested that the Rosebrae Beds were 'possibly as young as Tournaisian'. In the Midland Valley of Scotland the interval depicted on the map is represented throughout by Old Red Sandstone facies. Indeed, over the western part of the Midland Valley this facies persisted well into the Carboniferous (q.v.). Within the area there is little biostratigraphical control and correlation across the region is based on lithostratigraphical criteria and is thus of dubious value chronostratigraphicaUy. The upper part of the succession in the Firth of Clyde area, where the succession is thickest, belongs to the Leap Moor Formation (Bluck 1978) consisting of sandstone-siltstone-mudstone upwards-fining cycles. The beds are of various colours, but the mudstones are reddish or green in colour and contain calcrete horizons. The calcretes have been locally reworked into conglomerates also containing clasts of vein quartz and quartzite. A localized breccia with Dalradian clasts occurs along the Highland Boundary Fault on the northwestern margin of the Firth of Clyde; this interdigitates with the calcretes (Bluck 1980)~ These calcrete-containing beds are known as the Cornstone Beds in the region to the west of Stirling (Francis et al. 1970) and as the Kinnesswood Formation in the Lomond Hills, Fife (Chisholm & Dean 1974) and the Firth of Tay region (Browne 1980). Beneath the Kinnesswood Formation in the Lomond Hills is a white arkosic sandstone known as the Knox Pulpit Formation. Bimodal current indicators in this sandstone have suggested a possible marine incursion
(Chisholm & Dean 1974). Such an interpretation is not too difficult to reconcile with the palaeogeography depicted on the map, as the first incursions of the Carboniferous marine transgression recorded in the area were into east Fife, as on the mid-Courceyan map (q.v.). The evidence for this marine incursion is not unequivocal, and no data yet available from the North Sea reveal marine incursions into the area at this time. An alternative aeolian origin for the Knox Pulpit Formation has also been suggested. Outcrops on the southern side of the Midland Valley consist of sandstonesiltstone-mudstone successions, again with calcretes which are locally thick, particularly in Ayrshire, where they were formerly worked as a source of lime. JCWC, GDS
C2: Mid Courceyan The five million years which had elapsed since the start of the Carboniferous period shows a major transgression of the sea over the southern parts of the British Isles and the change from Old Red Sandstone facies to marine early Carboniferous facies was largely complete. This early Carboniferous transgression was more extensive than some later Dinantian ones, particularly over South Wales, where the Lower Limestone Shales are well developed over the north crop and in the Midlands, where the Midland-Wales High (St George's Land) was evidently extensively breached by a marine incursion from the south. This is clearly shown by the Lower Limestone Shales (and equivalents) with fully marine Courceyan faunas, such as those obtained from the Clee Hills and from Lilleshall, Shropshire (Mitchell & Reynolds 1981). How far northwards this marine facies extended is not clear, but the interpretation shown on the map is that it may have reached the site of the Widmerpool Gulf. The Hathern Borehole (Llewellyn et al. i969) shows well developed anhydrites within a mid-late Courceyan succession; this borehole lies on the southern margin of the Widmerpool Gulf. The map shows the red beds of the Caldon Low Borehole and the Duke's Wood Borehole as lying on the same alluvial plain with south-directed drainage (see above). The extent of this plain is conjectural, but it is based on the area of ground likely to be low-lying and extending southwards from the edges of the Askrigg Block and the Rossendale Block. The Peak District High is interpreted as high ground above this alluvial plain. Whether sedimentation at Duke's Wood and Caldon Low were part of the same system is at present unknown. The source of the anhydrites at Hathern is conjectural too. The sea could have entered the Widmerpool Gulf at this time from the west, as indicated on the map; on the post-Courceyan regression in this area a remnant saline lake would have been left. Alternatively, the anhydrites may be the product of the alluvial drainage system in which the alluvial beds of Caldon Low and Duke's Wood accumulated. In this case the hypersaline area of the Widmerpool Gulf need not have had connection with the sea to the immediate west. To the south of St George's Land, the boreholes at Cambridge and Gayton (Worssam & Taylor 1969) are interpreted as lying on the southern edge of the landmass. All known occurrences to the south of this (Aston Tirrold, Warlingham and Harmansole) show fully marine successions. In North Devon, deposition of the Pilton Beds continued on from earlier times (q.v.) right across the northern side of the Culm synclinorium. To the south, the Yeolmbridge Formation of north Cornwall is succeeded by the Tintagel Group. The succession here includes a series of volcanic rocks which represent a period of short-lived explosive marine volcanicity (Freshney et al. 1972). The succession includes tufts together with some agglomerates and lava flows. Some earlier authors (e.g. Batstone 1959) have recorded pillow lavas, but the presence of these was not confirmed by the Geological Survey (see Freshney et al. 1972, p. 43). The precise age of the Tintagel Volcanic Formation is not known, but it seems to fit best into the mid-late Courceyan interval. In southern Ireland (Sevastopulo 1981) the transgression of the Courceyan effects the change from the fluviatile sandstones seen on the previous map through tidal flats, lagoons and interdistributary bays to shoreface sandbars and deltaic flats. These in turn give way to subtidal environments of marine sands and muds. The map records the beginnings of the northward spread of carbonate deposition across the area. In the South Munster Basin (south of the Cork-Kenmare Line) thin limestones succeed the subtidal deposits abruptly and the presence of phosphatic nodules (including some showing evidence of rolling) suggests a non-sequence at this point. The establishment of this open marine environment is seen across the area with the exception of the 'Glandore High', an area extending from Dunmanus Bay to the Seven Heads, which appears to have received little sediment at this time, but which was almost certainly beneath the sea. The Courtmacsherry Formation in the eastern part of the basin corresponds with the Reenydonagan Formation in the west; both contain a fauna of brachiopods, bryozoa, solitary corals and conodonts. Northwards from the Munster Basin, the succession thins, although there is a local thick development in the Shannon Trough. The succession between Kenmare and Tralee in the west, and Cork and Dungarvan in the east is dominated by limestones. Further to the north again is a wide belt of limestones (often with hematite) and shales which, like the facies in the Munster Basin, succeeds intertidal flat sediments with a sharp break. As in the former area too, the basal beds contain phosphatic material and an open marine fauna. The facies associations in Ireland display a deepening of the water southwards, and although the facies must be to some extent diachronous, a
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
70
CARBONIFEROUS
northward transgression seems clear. On this basis the littoral sedimentation seen to the south in earlier Courceyan times must have been represented by a similar facies belt to the north and east of the limits of marine deposition at this time. Recent work on the miospore assemblages by Higgs et al. (1988) has allowed the area north and east of the limits of fully marine deposition to be correlated with the succession farther to the south. This work again shows the northward prograding of the marine transgression, the youngest Old Red Sandstone facies being well up in the Courceyan (or higher--q.v.) in some places. It is clear that miospore assemblages provide the best means of correlation from non-marine through to marine facies. On the map, the area shown as yellow-green covers the region in which the BP/PC miospore zonal boundary seems to fall in beds of intermediate facies between non-marine Old Red Sandstone and fully marine Carboniferous. In the Wexford area the limits of this zone of intermediate facies are well constrained, but successions to the north at Tynagh and Slieve Aughty still show Old Red Sandstone facies at this time. From the Shap region of northern England, Kimber (1984) has described a series of elastic formations which he believed extended northeastwards under the Vale of Eden. Leeder (1974) has reconstructed the drainage system off the Southern Uplands. At this time the drainage probably ran in an eastnortheasterly direction along Leeder's 'Northern Gulf' as an extension to the south (Leeder's 'Solway Gulf') seems palaeogeographically less likely. Over other parts of northern England there is no direct evidence of rocks of this age, but they may be present in the subcrop of the Craven Basin. To the east, the margin between non-marine deposition and land undergoing erosion is not well established and has for convenience been taken along a line extending southeastwards to the Dowsing Fault. By mid-Courceyan times it seems unlikely that the change from Old Red Sandstone facies to the cementstone facies, characteristic of the beds traditionally regarded as basal Carboniferous in Scotland, was complete. Over much of the Midland Valley in the time interval depicted on the map Old Red Sandstone facies of the Kinnesswood Formation continued to be deposited. (Lithological details may be found above on p 68.) Palaeocurrent indicators suggest an alluvial system draining eastwards. Desiccation cracks, gypsum and salt pseudomorphs suggest sabkha environments. In East Fife and East Lothian, the oldest Carboniferous rocks exposed are of latest Courceyan age, but it seems clear that in mid Courceyan times the area must have been close to the western limits of the occasional incursions from the sea which lay at some distance to the east. Such marine horizons may be preserved at depth in the area and the map suggests that the eastern part of the Midland Valley lay within this realm of periodic marine incursions. Such marine incursions are more likely over the North Sea region to the east, but further to the north the Buchan Field and Well 44/2-1 record no marine horizons at this time. In the Clair Basin (Meadows et al. 1987) the lower part of the succession, which has not been dated accurately, has yielded holoptychian scales of Tournaisian aspect (Ridd 1981). In the Southern Uplands and the English border region, the lavas of Birrenswark and of Kelso probably extended up to this time. JCWC
C3: Latest Courceyan-early Chadian The interval here depicted covers a slightly broader spectrum of time than that of many of the maps, since the interval chosen is not particularly diagnostic biostratigraphically. Over southwest England deeper water sediments continued to accumulate south of the carbonate shelf which existed over many areas to the north by now. In Cornwall the Tintagel Volcanic Group is probably largely of this age. The succession consists predominantly of basaltic lavas, but there are also tufts and agglomerates and possibly some pillow lavas interbedded with the slates, although the presence of pillow lavas has been disputed (Freshney et al, 1972, p. 43). In the area around Launceston is a succession of volcanic rocks which include pillow tavas claimed by Chandler & Isaac (1982) to be of ocean floor affinity, associated with other lavas and volcanogenic sediments. These authors were able to establish the age of the rocks as latest Courceyan to mid Vis6an, whereas the rocks had previously long been regarded as Devonian. In the Okehampton area (Edmonds et al. 1968) occurs a succession of quartzi'tes interbedded with shales (the Meldon Shale and Quartzite Formation). Within this formation is intercalated a series of volcanic rocks including thick tuff beds and agglomerates and known as the Meldon Volcanic Beds. To the east of here, in the Exeter-Chudleigh area, a succession of siliceous shales accumulated which have yielded conodont faunas of this age (Matthews 1969). In North Devon the highest part of the Pilton Beds, in the Fremington and Tavistock areas, is of this age. To the east of these localities, from the region of Barnstaple eastwards, a cherty facies is developed, known as the Bydown Chert (Prentice & Thomas 1960). Across the north crop of the South Wales coalfield area a major break occurs in this part of the Carboniferous succession; in many areas Chadian and Arundian are missing or very much reduced in thickness. Palaeokarstic surfaces have been recorded from many areas in South Wales and also from the Bristol area, showing that there must have been a broad regional uplift at this time. Movement along the Severn Valley Fault Zone also seems probable (Wilson et al. 1988).
In south Pembrokeshire (southwest Dyfed) Waulsortian facies (q.v.) occurs in this interval, as it does also in the Knap Farm Borehole at Cannington Park (Whittaker & Scrivener 1982). The occurrence of the Waulsortian facies in both west Somerset and southwest Wales at this time strongly suggests that a belt of Waulsortian facies may have been present along the line of the Bristol Channel at this time (roughly paralleling the depositional strike). Extension of this carbonate platform eastwards is shown by the boreholes at Aston Tirrold (Foster et al. 1989) and Warlingham (Worssam & IvimeyCook 1971). How far north the platform extended onto the southern margin of St George's Land is unknown, and the shoreline shown on the map is not drawn with any degree of certainty. Compared with the previous map, the late Courceyan shows a rapid transgression of the sea across the Irish Midlands and into Northern Ireland. Much of the area shown, over the southern half of Ireland, north of the Cork-Kenmare Line is depicted as reef limestones on the map. These are the Waulsortian Mudbank Limestones of Sevastopulo (1981). The Mudbank Limestones are fine-grained grey limestones forming mounds or sheets with original depositional dips of up to 40 ~ These limestone banks were up to c. 15 m high and a few hundred metres across and interdigitated or coalesced with contiguous mounds. Between the Mudbank Limestones were thinbedded cherty or shaly limestones, the whole forming a distinctive facies association which Sevastopulo (1981) termed the Waulsortian Limestone Complex (see also important papers by Lees et al. (1985) and Lees & Miller (1985)). The fauna, in addition to common brachiopods, includes a rich variety of molluscs including goniatites, nautiloids, gastropods, bivalves and rostroconchs. Together with these are trilobites, ostracods, bryozoans, crinoids and blastoids. South of the Cork-Kenmare Line is a largely argillaceous succession, the basinal Reenydonagan Formation, some 10 m of black mudstones and some thin limestone turbidites derived from the northeast (Jones 1974). In the southeast, the landmass forming the western end of the St George's Land high is flanked by a poorly known Courceyan limestone succession, but apparently devoid of Waulsortian facies (Sevastopulo 1981). Southeast of Galway, sandstones intercalated with non-Waulsortian limestones were probably derived from the land area to the northwest. Further north in Ireland, the late Courceyan rests directly on the Lower Old Red Sandstone or on older rocks. In the Glenoo Borehole (Sheridan 1972) basal sandstones are overlain by a sequence of shales and evaporites and these succeeded by a predominantly limestone/shale sequence. The basal sandstones are seen to crop out to the north in the Omagh Syncline, where they again rest directly on the basement. In the Kingscourt Outlier, bedded bioclastic limestones form the upper part of a somewhat thinned succession resting on the basement of the Lower Palaeozoic rocks of the LongfordDown massif. From Clare Island Higgs et al. (1988, p. 43) recorded an assemblage of CM and early Pu Biozones, the latter showing some evidence of marginal marine conditions. This occurrence has important palaeogeographical implications, as the locality is situated very close to the margins of the GalwayMayo High and suggests that there was an embayment in the eastern side of the high, along the line of the Highland Boundary Fault. There is, however, no suggestion from this region that the fault was active at this time; the embayment may merely reflect more active erosion along the fault. At Draperstown an assemblage of miospores was recorded by Higgs et al. (1988, p. 44) which indicated probable Pu Biozone. The rocks here are in typical Scottish Midland Valley Cementstone facies suggesting that the area lay on a southwestward extension of the Scottish depositional basin, which was not separated from the marginal marine rocks to the southwest. The earliest transgression in North Wales onto the northern flank of the St George's Land high has recently been shown to be of Chadian age (Davies et al. 1989, Somerville et al. 1989). The underlying Carboniferous basement beds could be as early as Courceyan. These red elastic beds are overlain by dolomitized carbonates which accumulated in near-shore high-energy environments. Somerville et al. (1989) suggest the possibility that Waulsortian mudbanks could well have accumulated offshore, north of the present-day coast. The local depositional margins on the St George's Land high were identified by these authors as the Vale of Clywd Fault and the Bryneglwys Fault (the latter is part of the Bala Fault System) as earlier suggested by George (1974). It is thus clear that the northern margin of St George's Land was experiencing transgression at this time--a feature which accords well with the successions in Ireland and in the north of England. This view is at odds with the record from South Wales and the Bristol area which seems to require an extension southwards of the southern margin of the St George's Land high. It seems clear from the evidence from both the north and the west that the apparent regression in the south of the area can only be due to a local upwarping at this time. In the Peak District Waulsortian facies occurs along the StaffordshireDerbyshire borders. The Eyam Borehole (Dunham 1973) showed, at the base of the Carboniferous succession, sandstones with anhydrites; these are here identified as a sabkha of late Courceyan-early Chadian age (recorded as C, by Dunham, 1973). Beneath the Carboniferous were possible Lianvirn rocks. The occurrence of an anhydritic sabkha here is interesting and probably correlates with the upper part of the anhydrite succession in the Hathern Borehole (q.v.). There is also mention of anhydrites at depth over an area extending from Eyam southeastwards as far as Mansfield, some 40 km away (Dunham 1973), suggesting a significant area of sabkha facies
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
72
CARBONIFEROUS
extending over the East Midlands shelf to the land to the south and east-the land forming the northern margin of St George's Land and extending northwards across east Lincolnshire and Yorkshire as far north as the Alston Block. In Northumbria, a cementstone facies, similar to that of the Scottish Midland Valley, persisted through to this time with occasional marine incursions probably reaching as far south as the southern margin of the Alston Block. Leeder (1974) interpreted these as being deposited on the coastal plains of a river system to produce the Bewcastle delta. Leeder (1974) also traced the evolution of the drainage off the Southern Uplands (see also Deegan 1973). Compared with earlier Courceyan times, relief was reduced and the Whita river system and alluvial fans farther to the southwest discharged smaller amounts of sediment into the Solway Basin, reflecting the by-now more subdued relief of the Southern Uplands block. The Solway Basin was by now a marine-influenced gulf in which carbonates and shales as well as paralic sandstones were accumulating. The boundaries of the Solway Basin are probably faulted; Barrett (1988) has discussed possible faulting of the southern margin beneath the Permo-Triassic sediments. Drainage in this area now appears to be reversed following the advance of the marine transgression from the southwest and the influence of the prograding delta from the northeast. To the south of the Solway Gulf the Lake District-Manx High was separated from the Alston Block by an area of non-marine sedimentation extending from the Vale of Eden southwards to the Shap area. The Shap Red Beds have been described by Kimber (1984). They include conglomerates derived from Silurian greywackes to the south and southwest; these grade up in a succession of fining-upwards cycles into cross-bedded sandstones. Similar red conglomerates at Sedbergh to the south are proximal deposits which were derived from the northwestern side of the Askrigg Block. The Pinskey Gill Beds are at the base of the Carboniferous succession at Ravenstonedale in the Stainmore Trough; these are peritidal carbonates of mid-late Courceyan age. Their conodont faunas were studied by Higgins & Varker (1982). The Swinden No. 1 Borehole (Charsley 1983) showed a succession of carbonates and fine-grained terrigenous sediments of shallow water origin. They form a single formation which, Charsley concluded, was deposited rapidly. The borehole, although in the Craven Basin, lies close to both the Askrigg Block and the Rossendale Block, but there is no indication of which (if either) of these areas provided the terrigenous sediment source. Later, Waulsortian facies developed in the Craven Basin with the richly fossiliferous :reef knolls' of the Clitheroe area. The interval depicted on the map corresponds to the period immediately preceding the major volcanic episode which produced the Clyde Plateau and other lavas (q.v.). In the western part of the area the Ballagan Formation is part of the Cementstone Group. The cementstones are fine-grained impure dolomitic limestones, which occur in thin beds interbedded with mudstones. Evaporitic conditions obtained in many areas. The fauna of the Ballagan Formation consists mainly of non-marine bivalves and calcareous worm tubes. Correlation of these non-marine beds is again difficult. In West Lothian the cementstones are interbedded with a predominantly arenaceous sequence with some calcretes. To the east, lower horizons are not seen at outcrop. In East Lothian the Spilmersford Borehole (Neves et al. 1976) shows shales and mudstones of the Pu miospore biozone beneath a volcanic succession. These beds have been correlated with a succession showing occasional marine influences which now spread further west in the Midland Valley, extending beyond Edinburgh and Dunfermline and which may have reached almost to the Stirling area. JCWC
C4: Arundian Following the widespread occurrence of Waulsortian facies in the late Courceyan to early Chadian, the facies became much more restricted during the Chadian Stage and by the Arundian had disappeared from the area depicted on the map. In North Devon the deposition of muds and cherts across the area continued in the deep water trough, whose margin must have lain just to the north of the present outcrop of the Carboniferous rocks. The edge of the carbonate platform could only have been a short distance to the north. Arundian faunas have been identified along the strike from Tavistock to Bampton (Prentice 1960, 1967). To the south, in the outcrop running from south Devon across to the Launceston area of Cornwall, muds and sands of the Meldon Shale and Quartzite Formation continued to be deposited. The formation appears to bridge most of the interval from late Courceyan through to Brigantian, but has yielded no diagnostic faunas in the area. It seems at least possible that this formation could include one or more non-sequences, but until some biostratigraphical information is forthcoming this can only be surmised. In the Boscastle area. the Buckator Formation (Freshney et al. 1972) consists of green slates with at least two limestone horizons; the whole occurs as thrust slices in the Namurian Crackington Formation. In such a complex tectonic setting it has not hitherto been possible to determine the thickness of the succession, but it may not exceed 30 m (Freshney et al. 1972. p. 20). The lower limestone may be of Courceyan age according to the sparse fauna of brachiopods and corals (Freshney et al., op. tit. p. 32) but conodont faunas give a younger date which is equivalent to the German Cu II~5of late Asbian age (George et al. 1976. p. 75). The facies may be condensed or there
may be thrusting, but if the macrofossil and conodont ages can be reconciled (could the macrofossils be reworked?) then rocks of Arundian age may also be represented in the Buckator Formation. In South Wales and the Mendip Hills area the uplifted surfaces of Courceyan limestones which had been subjected to karstic weathering during Chadian times (q.v.) were inundated once more by the sea and a succession of peritidal carbonates accumulated to the north of an oolite barrier (Wilson et al. 1988). To the south of this barrier was an area of open marine facies. Displacement of the facies pattern when comparing the successions across southeast Wales with those of the Bristol/Mendips area led to the suggestion that a Severn Valley Fault Zone had displaced these facies patterns and that there had been syndepositional faulting along this line (Wilson et al. 1988) as in the Courceyan (q.v.). Arundian rocks are not known over an area extending eastwards from the outcrops of South Wales and the Bristol/Mendips area. It seems likely that the Arundian rocks were originally deposited over this area and subsequently stripped. Part of this erosion of the Carboniferous Limestone succession may have been through penecontemporaneous karstic erosion, such as that which occurred in Chadian times in areas to the west (q.v.). That such karstic erosion did occur in southeast England may be attested to by the only limestones of Arundian age known in the area. The Penshurst Borehole (Dines et al. 1969) yielded, from near the base of the borehole, brecciated limestone with a fauna indicative of a position close to the Chadian/Arundian boundary. Palaeogeographical considerations suggest that an early Arundian age is the more likely. The South Munster Basin or Ireland was largely starved of sediment and a succession of highly condensed, mainly mudstone, facies accumulated. Some of the mudstones are cherty, as around Bantry Bay, where much of the Arundian (together with all of the Chadian) totals less than 50m in thickness. In this latter area some fine-grained limestone turbidites also occur, derived from the carbonate shelf to the north. North of the South Munster Basin onto the shelf, much of the Irish succession consists of regularly bedded bioclastic limestones, although there is some local variation. In northern Co. Cork the early part of the Arundian contains some tufts, the derivation of this material may be from the north, where, in the area between Limerick city and Tipperary town, basaltic lavas, tufts and agglomerates occur (Sevastopulo 1981--see also for earlier references). The main focus of this volcanic activity appears to be Chadian and Asbian rather than Arundian. Further to the west, bedded limestones pass laterally in northwest Co. Limerick (but still south of the Shannon) into a much more argillaceous facies, the Rathkeale Beds, of Chadian and Arundian ages. Northwards from here into northern Co. Clare and southwest Galway are more bioclastic limestones; the base of the Arundian is marked by peloidal limestones possibly indicating local shallowing. In the area to the west of Dublin and across the Irish Midlands the Arundian succession consists predominantly of dark fossiliferous basinal limestones and shales, often cherty. In the Dublin area itself, in the coastal succession at Rush, the base of the Arundian is marked by the Carlyan Limestone, an oolite with horizons containing exotic clasts. North of the basinal succession of the Irish Midlands the Arundian consists of carbonates, locally argillaceous, but with developments of a marginal facies towards the edges of the depositional basin. A marginal sandstone facies, originally named the Calp Sandstone by the Geological Survey in the last century, has more recently been accorded various local formational names (see SevastopuIo 1981 for references). This marginal facies extends across country from the Omagh Syncline via Donegal and Sligo into northern Mayo. The age of these sandstones is not known with certainty, but they lie within the Arundian/Holkerian interval and are here shown on the Arundian map. In western Sligo the sandstones contain much plant material, both in situ and drifted, the latter presumably being derived from the adjacent Galway-Mayo High. On the map Clare Island is shown as lying within the marginal facies belt, this extension is based on the record of Higgs et al. (1988) of early Pu miospore biozone here, suggesting that later deposits could well have been present (see p 72). The map shows clear evidence of the transgression of the sea into northern England from Ireland. As, however, it is presumed that there was now a marine connection to the east of England, as depicted on the map, a transgression from that area cannot be totally precluded. By this time the margins of both the Alston and Askrigg blocks were being encroached upon by the sea. The conodont faunas from Ravenstonedale recorded by Higgins and Varker (1982) show increase in water depth during the Arundian, in line with a picture of continuing transgression. To the southwest of the Askrigg Block, limestone turbidites accumulated in the Craven Basin. Further to the south, marine conditions were established across the Peak District high as established by the Woo Dale Borehole (Cope 1973) and ended evaporitic conditions which had previously existed to the southeast (q.v.). The small islands shown on the map have been located geophysically and follow the palaeogeographical reconstruction of Strank (1987). In North Wales the Arundian limestones overlapped the earlier Carboniferous sequence and in places overstepped farther onto the Lower Palaeozoic northern margin of St George's Land (Davies et al. 1989; Somerville. Strank & Welch 1989). Records so far released show that areas offshore from 'the Scottish Highlands (e.g. the Buchan Field) have no marine intercalations, whereas to the east of the area south of the Highland Boundary Fault successions are predominantly of deltaic sediments, sourced from the north, with coals
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
74
CARBONIFEROUS
developed locally. There are occasional marine incursions across this area and it is presumably some of these which are recorded in the onshore successions in the eastern part of the Midland Valley of Scotland. Further to the south, boreholes off the Norfolk coast have recorded shelf limestones of uncertain early Carboniferous age; these may include Arundian carbonates. The picture thus appears of a marine depositional area to the south in which carbonates were deposited, passing north to a delta system showing occasional marine influence; this in turn passes northwards to alluvial plain sediments devoid of marine influence. The Clair Basin may, however, complicate the picture, as a marine incursion there may he of this age. Sedimentary studies of the Clair Basin (Meadows et al. 1987) have suggested derivation of sediment was from high ground to the north and west of the basin. Indeed, Hitchen & Ritchie (1987) speak of a montane environment to the northwest of the Clair Basin. However, an interval of marine sedimentation of undifferentiated Vis6an age occurs in the basin and may be of Arundian age. That the ground to the north and west seems unequivocally high may preclude a marine incursion from the western side following Haszeldine's suggestion (1984) of a marine rift to the west of Scotland. It may well be that publication of unreleased well data sheds light on the direction from which the Clair Basin marine incursion came, but it seems at least possible that there was a marine incursion, albeit brief, over the area of the northeastern corner of the map, for which no details are yet available. In the Midland Valley, the Arundian was a time of major volcanic activity, Eruption of the Clyde Plateau Lavas in the central part of the Midland Valley, which began in the Chadian, effectively divided the Midland Valley into two parts. In the southwestern part of the Midland Valley sedimentation of the Cementstone facies continued in an area almost entirely physically separated from the area to the east. No marine incursions are known in this southwestern area. The Clyde Plateau Lavas themselves are known to extend eastwards, beyond their present outcrop in the Glasgow region. Eruptions were mainly from ephemeral central volcanoes, sometimes preserved as vents less than 500 m across. Note that the centres shown on the map in this area are not taken to represent specific volcanoes which were numerous and short-lived. The lava sequences built up above the levels of the alluvial plain and so are virtually devoid of interbedded sediments. In Renfrewshire the lavas reach a maximum thickness approaching 1000 m, whilst farther east around Stirling the thickness is closer to 700 m. In the Edinburgh area the Arthur's Seat volcano and the lavas of the Garleton Hills in East Lothian are also of this age (Francis 1991). The eastern part of the Midland Valley came under increasing marine influence at this time, although the sediments are still predominantly nonmarine. The occasional marine incursions continued to extend westwards, probably reaching as far west as the rising pile of Clyde Plateau Lavas. Correlation around this interval is still difficult though and biostratigraphical precision is lacking. In Fife and East Lothian a few thin coals are present in the succession and this facies is seen as the westward extension of a facies becoming increasingly known from the North Sea. JCWC
C5: Brigantian By Brigantian times extensional tectonics had completed the creation of a series of half-grabens north of the St George's Land high. On the submerged highs and blocks carbonate precipitation continued to build up, but in the basins thin shaly sequences accumulated beyond the reaches of carbonate sedimentation. Thus the depositional patterns reinforced the sea-bed topography which was initially tectonically determined. Across the southwest England synclinorium, basinal deposits of Brigantian age have been recorded. Over much of the area, as on the north Cornish coast succession around Boscastle, the stage is represented by a cherty facies with goniatites. These cherts (the Fire Beacon Chert Formation) are represented further to the east, in the Okehampton area, by the Meldon Chert Formation. In the Chudleigh and Exeter areas the deposits are chiefly slates with some limestones. The most southerly areas of Brigantian rocks are in south Cornwall, where the Crocadon Formation consists of shallow water deltaic facies including conglomerates with clasts of igneous rocks, chert and shale together with horizons yielding rootlets. Miospores suggest the strata may range up into the Namurian, but the faunas of goniatites, ostracods and conodonts indicate a Brigantian age (Whiteley 1984). The formation provides evidence of the rising Normannian High to the south. On the north side of the synclinorium, the Brigantian is represented by cherty beds in the Barnstaple/Bideford area; here late Brigantian has been identified near the top of the Codden Hill Chert Formation (Riley in Edmonds et al. 1985). On the northeastern margin of the deep water trough of the southwest England synclinorium, limestone turbidites accumulated as the Westleigh Limestones, clearly derived from the carbonate shelf to the north. Around the Mendip Hills the Brigantian is largely absent and the Namurian (or later rocks) rests on Holkerian or Asbian limestones. In the Bristol area a succession of sandstone units, the Upper Cromhall Sandstone, dominates the Brigantian succession. In South Wales, Brigantian limestones occur widely over the south crop, but developments of this stage are generally thin when compared with other parts of the lower Carboniferous succession in the area. Ramsay
(1989) has interpreted the Brigantian outcrops of South Wales as lying on either side of a lagoon with tidal channels. Palaeosols are widely developed and these are envisaged as developing on prograding barrier islands. Palaeokarst surfaces are abundant on the seaward side of the islands, as in south Gower. The lagoon was closed at its eastern end by a high along the line of the Usk anticline. Brigantian rocks are unknown across much of southeast England; the rocks may have been originally deposited and subsequently stripped from the area. Sea levels were higher in the Brigantian than for some time and deposition over the region could thus be expected; the area is therefore shown in blue on the map. The only borehole to show Brigantian rocks in the area was at Kettering Road, presumably fairly close to the southern margin of the St George's Land barrier. To the southeast Brigantian limestones are known from the Boulonnais area of France. In the South Munster Basin of Ireland the Brigantian stage is represented by a thin succession of mudstones with some thin limestones totalling less than 60 m thick. They yield goniatites at several horizons. The succession, known as the Lispatrick Formationl is best developed at the Old Head of Kinsale. Passing from the South Munster Basin to the northern parts of Co. Cork, the Brigantian is represented by dark limestones, the Liscarroll Formation. Farther to the east, the successions are not well known, but around the Leinster and Slieveardagh Coalfields bioclastic limestones of Brigantian age become cherty at the top. In northern Co. Clare and southwest Galway both the Asbian and Brigantian successions are cyclical limestones (see Walkden 1987) and the platform on which the facies accumulated may have been periodically emergent, as at the Asbian/Brigantian boundary. South of this area the Shannon Basin shows a development of basinal limestones and shales with turbidites, known as the Inistubbrid Beds; similar facies extend across the Shannon to the NW part of Co. Limerick on the south side of the Shannon Basin. In the Dublin region, basinal facies are again well developed in the Brigantian. Turbiditic limestones and shales are interbedded with limestone boulder breccias and conglomerates; the faunas include Brigantian (PI) goniatites and the epi-planktonic bivalve Posidonia. The overlying black shales yield P2 age goniatites and appear to be alternations of decalcified cherty limestones and shales. The shelf/basin boundary to the north of the Dublin Basin is a complex slumped succession of Asbian-Brigantian grey micrites with conglomerates including clasts of Lower Palaeozoic rocks. These accumulated on a southerly directed palaeoslope. Jones et al. (1988) have suggested that faulting defined both northern and southern margins of the Dublin Basin. In the Kingscourt Outlier are thin limestones and black shales with goniatites, whereas just to the north of the basin boundary, around Strangford Lough small outliers preserve shelf limestones with nautiloids. Over the northern regions of Ireland a marked change occurred in late Asbian times, with the development of very shallow water environments which continued on through the Brigantian. In the Lough Allen area there is evidence of a supratidal sabkha with dolomites, desiccation cracks and evidence of evaporites in the late Asbian. In the Brigantian the succession is largely non-marine with coals in the northeast (the White Mine Coal yielding miospores characteristic of the VF Biozone) although marine horizons with Brigantian goniatites occur in the northwest. The Magilligan Borehole on the northern coast yielded thin coal seams and spores of late Dinantian age; they are considered here to be most likely to be of Brigantian age. To the east of this point at Ballycastle in Co. Antrim sandstones and conglomerates are overlain by lavas and tufts in a succession like that of the adjacent Machrihanish area of Kintyre. By Brigantian times it seems likely that basinal sedimentation was continuous from the Dublin Basin to the Craven Basin and thence to the Gainsborough and Widmerpool gulfs, the latter with Brigantian turbidites. Shelf limestone facies was now widespread across the East Midlands Shelf (Strank 1987). In Derbyshire reefs were developed within the carbonate platform. The presence of a barrier to the south must be questioned although the Whittington Heath Borehole, with only 0.75 m of Brigantian rocks, and the Rotherwood Borehole with Brigantian anhydrites, suggest the barrier may have still existed--at least for part of the time. Volcanicity occurred at this time in the Isle of Man, the Peak District and the Little Wenlock area of Shropshire. In North Wales the shoreline had advanced considerably since Arundian times. Although the Holkerian had seen a transgression compared with the Arundian, it was not until Asbian times that the transgression advanced far over the basement. Asbian beds rest directly on Lower Palaeozoic in the Corwen area and farther to the northwest in the Llandudno and Anglesey areas. The Brigantian contains cherty horizons and in the Llangollen and Oswestry areas sandstones are developed within the limestone succession. The marine transgression of the Askrigg Block which had begun in the Arundian (q.v.) proceeded through the Holkerian and more particularly the Asbian. By Brigantian times, reefs were developed on the southern margin of the Askrigg Block. During the Asbian the Alston block too was inundated. During the Brigantian both areas saw the development of the Yoredale Facies across the region. This facies consists of cyclothems beginning with a marine limestone through a coarsening-upwards succession of shalesiltstone sandstone indicative of delta progradation, ending with delta top formation of a thin coal. Leeder (1978) has discussed the origin of the Yoredale cyclicity and suggested that a combination of tectonic and purely
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
76
CARBONIFEROUS
sedimentary (autocyclic) processes were involved and locally overrode the importance of glacio-eustatic sea-level changes whose importance regionally was very great. Walkden (1987) has discussed the cyclicity of carbonates in the late Dinantian. These may involve emergence and sometimes the development of palaeosols. He has concluded that eustatic sea-level changes, possibly related to the Gondwanan ice cap, were the dominant factor although tectonic subsidence was also to be taken into account. Offshore from northern England and the Midland Valley of Scotland, Yoredale facies accumulated over a wide area of the North Sea. Borehole data so far released across the region show widespread development of coals at this time and, as these are potential source rocks for gas, there is considerable exploration of the area now taking place. Coals also occur frequently in the alluvial facies further to the north, where no marine incursions are yet known. The single marine horizon within the Vis6an of the Clair Basin may be of Brigantian age. For discussion of this and the origin of the marine incursion see the description of the Arundian map C4. Marine influence continued to increase in the Midland Valley of Scotland, and although the rocks are still predominantly non-marine, by Brigantian times reefs were present in the central part. By this time too, marine horizons do at least provide a means of correlation across the whole of the Midland Valley from Ayrshire to East Fife. The upper part of the Calciferous Sandstone measures is of Brigantian age and it constitutes about half :the thickness of the Brigantian; the upper half belongs to the overlying Lower Limestone Group. The upper part of the Calciferous Sandstone Measures includes the Upper Oil Shales Group--a succession of shales, locally bituminous, interbedded with sandstones. Together with these are occasional thin limestone bands, some of them of marine origin, and thin coals which tend to be laterally impersistent. In Midlothian the Upper Oil Shale Group reaches its maximum thickness of 850 m (Cameron & Stephenson 1985) although only the upper part is of Brigantian age, and contains up to ten oil shales which were worked commercially. Three marine horizons occur in Midlothian in the Brigantian part of the succession, but in East Fife marine incursions were more frequent and are present throughout the Brigantian. The Lower Limestone Group consists of up to 220 m of sandstones and mudstones together with common limestones. Marine horizons achieve greater importance at this time and are correlatable widely across the Midland Valley, yielding goniatites of Brigantian age (Currie 1954). Miospore work by Neves et al. (1973) identified the base of the VF Biozone at a short distance below the base of the Lower Limestone Group in East Lothian. Variations in thickness of the Lower Limestone Group across the Midland Valley can be related to differential subsidence related to the existence of thick sequences of lavas in the underlying rocks, and to syndepositional movement along faults in the region (see also Read 1987). Coals within the Lower Limestone Group were occasionally thick enough to have been worked in the past. JCWC, GDS
C6 and C7: Namurian (Arnsbergian, Marsdenian) There is general agreement that Britain was equatorial during the Namurian (Smith et al. 1981; Scotese et al. 1979). The occurrence of coal and bauxitic soil horizons in Scotland indicates a humid, tropical climate. Britain was disposed to the southeast of the North American-Scottish-Scandinavian Caledonides, with an east-west trending foreland basin (Barnes & Andrews 1986) to the south. Faunal evidence (Ramsbottom 1970) supports the existence of sea connections east to the USSR and west to North America. The orogenic episode associated with the final closure of the proto-Tethys in south Europe later in the Carboniferous (Ziegler 1984) was to close the basin in the south of England and this complicates the palaeogeographical interpretation. A pervasive theme in the Namurian stratigraphic record is cyclicity; the pattern of thick 'Millstone Grit' sandstones alternating with siltstones and thin marine shales occurs in most areas. Because of the apparent lateral persistence of individual marine horizons, the cyclicity is thought to be eustatic in origin (Ramsbottom 1977) and a stratigraphy of'mesothems' has been devised (Fig. 1). Although sea-level changes may not have been great, the consequent changes in palaeogeography were extensive due to flat topography. Thus the coastline changes within one mesothemic cycle were greater than the general trend of coastline changes through the series. Ramsbottom (1979) argued that transgressions were pulsed, and complete transgressions occupied half of the I. 1 to 1.35 Ma of each mesothemic cycle. Holdsworth (1966), however, estimated an extremely slow sedimentation rate (1 cm per 1000 years, before compaction) for the marine black shales, which would allow the marine phases to occupy nearly all the available time. This is feasible in view of the potentially extremely rapid sandstone deposition and the paucity of coals, which need time to accumulate. Therefore, despite the low percentage thickness of marine shales, widespread marine conditions probably prevailed for the bulk of the Namurian. The dominant clastic facies of the British Namurian are in contrast with the carbonate-rich Dinantian, but this is not attributable to general regression (Namurian deposits overlap the Dinantian and the transgressive trend persists into the Westphalian); it relates instead to uplift of eroding
areas coupled with climatic change (Besly 1987). During regressive phases of eustatic cycles, fluvio-deltaic sandstones prograded and substantially occluded the marine environments. Coal swamps developed on delta tops in the north, and deltas were a source for turbidite flows into the deeper basins. Marine fossils are not usually preserved in the regressive .phases. Transgressions are typically represented by thin siitstones and black shales immediately overlying a seatearth (or analogous peak regressive facies) and show a succession of faunas corresponding to the increase in marine influence (Ramsbottom 1969, fig. 3). The transgressive acme is often represented by a black laminated shale with 'thick shelled' goniatites (including the zonal species) and pectinoid bivales (e.g. D u n b a r e l l a ) which are either epiplanktonic or adapted for dysaerobic conditions; the lack of unequivocal benthos suggests reduced oxygen levels. In near-coastal areas, however, brachiopod-crinoid-mollusc communities are developed locally.
I•ms
Zones
Stages
Nll
Gla.b
YEADONIAN
N10
R 2c MARSDENIAN R 2b
N9
R 2a
N8
Rlc
N7
Rlb
N6
KINDERSCOUTIAN
R la H2c
N5
H2a, b
N4
Hla,b
N3
E29
N2
E 2b
ALPORTIAN
CHOKIERIAN
ARNSBERGIAN
E 2s N1 Ela-c
PENDLEIAN
Fig. !. Namurian mesothems (NI-NI 1) in relation to goniatite zones (E-G) and stages. After Ramsbottom (1977). Reproduced by permission of the Yorkshire Geological Society.
In general, palaeogeographical features had an east-west trend, and this allowed the traditional division into 'provinces' which is followed here: Southwest England, South Wales, the Central Province, Northern England, the Midland Valley of Scotland and Ireland. These provinces are areas which received sediment in the Namurian, and were separated by inferred land, for example the Wales-Brabant Island (St George's Land), or by tectonic features such as the Craven Fault Zone. In Southwest England the Namurian is largely represented by the Crackington Formation (c. 1000m) which constitutes much of the 'Culm' of Devon and Cornwall. This terrain underwent considerable tectonic shortening in the Hercynian orogeny, so there is inevitable distortion in the map representation. The rocks are turbidites which, in the lower Namurian, are more proximal in the south, though the nature of the source area is unknown. In the upper Namurian, current directions in the Crackington Formation are towards the centre of the region with, for example, derivation from the west and northwest indicated by flutes and grooves at the type locality (Freshney et al. 1977). This is interpreted as the result of southerly derived turbidites flowing along the long axis of the basin, but northerly derivation is also likely since deltas prograded from the north of the region during the lower Westphalian (Westward Ho! Formation). The eustatic cyclicity is manifested as rhythmic alternations in the degree of proximity of the turbidites. Goniatites are common in concretions or on the bases of turbidites; benthic faunas are rare. Although more abundant and well preserved in the shaley, distal units of the Crackington Formation. goniatites seem to occur sporadically throughout, indicating consistent marine conditions. This is in contrast to the succeeding Westphalian Bude Formation, which has been interpreted as largely non-marine. The general aspect of the Namurian of the South Wales basin is similar to that of the Central Province, with alternating fluvio-deltaic sandstones and marine shales, totalling a maximum of 600 m. During regressive phases, deltaic encroachment proceeded centripetally from all sides of the basin (George 1970; Oguike 1969), except perhaps for a seaway to the SSW. Sandstones did not, however, reach the centre of the basin at Gower until Marsdenian times. The hinterland of the deltaic source to the.south of the outcrop in Glamorgan is unclear, as it is apparently constrained to lie between north Devon/Somerset and South Wales. Erosion and tectonic shortening may have destroyed any trace of sufficient land in this position.
77
78
CARBONIFEROUS
Alternatively, there may be strike-slip displacement on the Bristol Channel Fault Zone. There was tectonic influence on local sedimentation patterns. Thickness and facies of correlatable rock units, for example the G~a (Cancelloceras cancellatum) Marine Band, vary considerably in short distances. The Neath disturbance, in particular, appears to have influenced the sedimentological complexity in the late Namurian around Glyn Neath (including bars, tidal channels, lagoon, beach facies etc., Oguike 1969). Development of structural axes probably influenced the different extent of successive mesothems (Fig. 2). . ~ x D R UIDSTO N (~1~
_
N9
_
HAROLDSTON
N6
TWRCH VALLEY
~
.~
N31~9
N6
_
~
lo
2jo
km
Fig. 2. The estimated extent of various mesothems in South Wales. Mesothem Nl I covered the whole region. After Ramsbottom (1978). Reproduced by permission of the Geological Society. The shape of the South Wales basin is similar to the reconstructed coastlines for much of the Namurian. This is reflected in the unusually high incidence of benthic faunas, especially along the north crop (Ware 1939). The typical goniatite-pectinoid fauna also occurs, but is better developed away from the basin margins. To the east, little is known of the south margin of the Wales-Brabant Island. An isolated basin in the Bristol region includes lowermost Namurian sandstones and marine shales, and it is likely that the fluvio-deltaic/marine scenario can be extrapolated ESE to continental Europe. The Central Province extends from the north coast of the Wales-Brabant Island to the Craven Fault Zone. It is the classic area of the 'Millstone Grit', characterized by thick, laterally persistent, coarse fluvio-deltaic sandstones. The sandstones sometimes covered virtually the whole province during regressive maxima, especially later in the Namurian (e.g. the Rough Rock). Where coarse clastics are absent from a cycle, the regressive phase is often represented by unfossiliferous pyritic or phosphatic siltstone, the environmental interpretation of which is obscure. Sediments are derived both from the south and the north. The contribution from the north is generally dominant and becomes stronger through the Namurian (Ramsbottom 1969, fig. 10). The mineralogy of these sandstones suggests a provenance in the Scottish/Scandinavian Caledonides (Gilligan 1920; Leeder 1988) and transportation is thought to have been by major braided river systems trending NNE-SSW. The maximum Namurian thickness is 2000m (in the Barnsley area, Ramsbottom 1974) but there are considerable lateral variations (Fig. 3) due to differential subsidence and deltaic influence. These delineate a series of east-west trending gulfs or troughs, controlled by basement half-grabens (Lee 1988) which were developed during Dinantian extension. In most areas, sediment accumulation appears to have kept pace with subsidence, but locally a sea bed topography developed, or was inherited from the Dinantian, with sufficient gradient to generate turbidites (e.g. Mare Tor and the Duffield borehole, 'Widmerpool Gulf', Ramsbottom 1969).
The degree of subsidence affecting the Central Province did not prevail north of the Craven Fault Zone, where the succession is consequently thinner (c. 300 m). Over half of this is attributable to the Pendleian and Arnsbergian Stages; the Chokierian to Marsdenian Stages are particularly reduced relative to the Central Province. This must reflect the main phase of Namurian movement on the fault zone, during which the Alston and Askrigg blocks were buoyed up by underlying granites. The Yoredale-type limestone-shale-sandstone cycles continue from the Dinantian until Kinderscoutian times. The occurrence of limestone as the transgressive facies indicates lack of clastic material plus aerated conditions; as the north is the direction of the source for the bulk of the sediments of central England, this confirms the supposition that the basinal marine black shales of the Central Province also received little sediment and were deposited extremely slowly. The limestones are of shallow water facies, with a good benthic marine fauna and rare goniatites. Higher in the Namurian, only the strongest transgressions spread goniatite faunas over the Northern England blocks: Rib, Rlc, R2c and G~b zone species are recorded (Ramsbottom 1977). In regressive phases, deltas rapidly prograded over the area en route towards the Central Province. Delta top or fluvial conditions must have prevailed for a larger percentage of the time in northern England, and this is reflected in the increased occurrence of thin Namurian coal seams. The Scottish Namurian is broadly similar to that of north England, and is dominated by Pendleian and Arnsbergian limestone-shale-sandstone cycles, which form the Limestone Coal Group (=Pendleian) and the Upper Limestone Group (=Arnsbergian). The former contains many economic coals, with which are associated non-marine faunas. Marine benthic faunas occur occasionally throughout. Goniatites have been found in this lower succession (Currie 1954), but the Chokierian to Yeadonian ages of the higher Passage Group are established largely by spores. The Marsdenian Stage may be represented only by Lingula bands. A trend developed of reduction in marine conditions which continued into the Westphalian; this is interpreted as reflecting a reduction in activity of the Midland Valley rift. Clastic material was derived largely from the Scottish Caledonides to the north. The maximum sediment thickness is 1500 m (near Clackmannan) but there is great variation which was controlled by faults with a NE-SW Caledonian trend, and by differential subsidence between lava piles (Read 1988). The depositional surface was, however, never far from sea level. A diversity of marine and non-marine environments developed locally, due largely to the palaeogeographical influence of volcanism. Namurian basaltic volcanism in the Midland Valley was widespread and of continental rift-type. There was greater explosive phreatic activity than in the Dinantian. Lavas and tufts are intercalated with Limestone Coal Group sediments in North Ayrshire, West Lothian and Fife; pillow lavas occur in the Passage Group of Ayrshire. Deep weathering of the basalt during the Namurian has locally produced a bauxitic clay. In Northern Ireland, the Dungannon Coalfield has a Namurian succession analogous to that of Northern England, and it similarly changes to a thicker basinal sequence to the south. The Namurian record shows a gradual filling of the basinal areas, for example in County Clare. The base, overlying the Carboniferous Limestone, varies from the Pendleian to Chokierian Stages. Goniatite-pectinoid shales dominate the sequence until the Alportian Stage; this sequence is condensed and contains bedded phosphorites and cherts, thought by Hodson (1954) to have been deposited in quiet, stagnant lagoons separated by submerged ridges. From Kinderscoutian to Yeadonian times, extensive, thick (100 m) deltaic sandstones prograded southeastwards from a Caledonian landmass to cover substantial parts of Ireland, though the margins are not accurately known. Deltaic approach is marked by well developed slumped sandstones. The fluvio-deltaic units are cyclic and have been interpreted using the Mississippi bird's foot model (Rider 1969). ARHS WESTPHALIAN
Fig. 3. Isopachs (in feet) of the Namurian in the Central Province. After Ramsbottom (1969). Reproduced by permission of the author. Goniatite-pectinoid-bearing black shale is the typical marine facies. In the subsiding areas, the marine bands, as well as the sandstones, tend to be thicker, or are split into multiple leaves. Benthic faunas are unusual but occur near the north and south margins of the province (e.g. the Otley Shell Bed). There was minor basaltic volcanism in the east Midlands during the Namurian (Falcon & Kent 1960).
The Westphalian maps are the result of a review of the literature, together with discussions with colleagues in the coal and petroleum industries. It is difficult to improve on the work of Wills (1956), who produced detailed palaeogeographical and isopachyte maps of the Westphalian of the Midlands. Wills (1948, 1951, 1973) made several other important contributions to this field. Trueman (1946, 1947) also published generalized palaeogeographical maps of the Westphalian, and later (1954) edited a comprehensive review of the coalfields of Great Britain. Unfortunately, there has been little recent research that would allow the production of detailed Westphalian palaeogeographies of Britain, but Calver (1968, 1969) published palaeogeographical maps illustrating the distribution of faunas in the Westphalian marine bands. Ziegler's atlas (1982) included a generalized Westphalian palaeogeography and provides a valuable background. Information on the present distribution of the Westphalian at outcrop and in the subsurface in the southern parts of Britain is provided on the map compiled by Smith (1985). The distribution of the Westphalian in the southern North Sea is based mainly on the maps produced by Besly (1990) and Leeder & Hardman (1990). Sedimentation of the Westphalian of the British Isles took place in delta plain and alluvial plain environments, with marine incursions generally being rare, especially in the upper part of the Westphalian A and Westphalian D. The maps show the broad distribution of these environments. Westphalian deposits show rapid lateral and vertical variation which is not
79
80
CARBONIFEROUS
possible to represent on the scale of the maps. Lithologies generally consist of claystones, siltstones and sandstones with interbedded coals and seatearths, and occasional breccias and conglomerates. Beds containing nonmarine fossils, especially bivalves, are abundant, particularly in the Westphalian A and B. Fossil plants are common throughout the Westphalian. The marine bands, which represent marine transgressions across the lowlying delta plains, are often of wide areal extent (Calver 1968, 1969) and of probable glacio-eustatic origin (Leeder 1988). The stratigraphical framework on which these maps are based is that of Ramsbott0m et al. (1978), and any terms used are taken from that report.
C8 Westphalian A (Langsettian) The Westphalian A (Langsettian) map depicts the palaeogeography of nonmarine strata immediately preceding the Vanderbeckei Marine Band, and thus represents a generalized palaeogeography for the lower part of the Modiolaris Chronozone. The Vanderbeckei Marine Band is of wide lateral extent (Ramsbottom et al., 1978), and thus the strata underlying this horizon can readily be correlated between coalfields. The extent of this marine band is depicted on the map. The interval chosen falls within Palstage lb of Wills (1956), which he defined as strata included in the Communis, Modiolaris, and Lower SimilisPulchra Chronozones. The top of his Palstage l b is marked by the Aegiranum Marine Band. Wills (1956) referred to the rocks of Palstage lb as the 'Main Coal Belt' as they contain the largest number of seams, many of which are of sufficient thickness to be mined. The underlying 'Lower Coal Belt' (Palstage la), which roughly corresponds with the Lenisulcata Chronozone, contains only a few seams which are locally workable, with several interbedded marine bands in many coalfields (Ramsbottom et al. 1978). The depositional environment of the upper part of the Westphalian A (Langsettian) of most of the exposed coalfield areas of Britain, north of the 'Variscan Front', has generally been interpreted in terms of upper delta plain and low gradient alluvial systems (Kelling 1974; Heward 1976; Guion 1978, 1984, 1987; Kirk 1983, 1989; Fielding 1984a,b, 1986; Fulton & Williams 1988; Guion & Fielding 1988; Read 1988). Important elements of the depositional environment include distributary channels and shallow, fresh to brackish lakes that were filled by a variety of mechanisms, including lacustrine deltas, crevasse splays and overbank deposition. Gradients were low and thus palaeocurrents in many cases represent local depositional conditions rather than regional palaeoslopes. However, in certain areas, contemporaneous tectonic activity influenced the pattern of sedimentation as detailed below. The palaeogeography south of the final position of the Variscan Deformation Front is difficult to construct meaningfully, as there has undoubtedly been N-S tectonic foreshortening and possibly strike-slip movement. No palinspastic restoration has been attempted. The Westphalian A of southwest England is represented by both the Bude Formation and the Bideford Formation (Edmonds 1984). However, the environments, are still controversial (Thomas 1988). Elliott (1976) interpreted the Bideford Formation as the deposits of a series of fluvialdominated deltas, with offshore turbidites. The Bude Formation was interpreted by Higgs (1984, 1986) as a thick Westphalian succession deposited mainly above storm wave base in a fresh-brackish lake with deeper water to the south, adjacent to the Variscan Mountain Front of the time. However, Melvin (1986) advocated deposition on a broad, gently sloping submarine fan, fed directly by delta systems. Palaeocurrents in the Bude Formation flowed from all quadrants except the south and thus a northerly source must be envisaged (Edmonds et al. 1979; Thomas 1988). Southwest England underwent orogenic compression soon after deposition, resulting in foreshortening to the present outcrop area. Several authors have advocated a Bristol Channel landmass (e.g. Owen 1971; Kelling 1974) to explain palaeocurrents from the south in South Wales and from the north in Devon. However, it is difficult to envisage how a relatively narrow palaeogeographical feature could have contributed large volumes of sediment to the Cornubian and the South Wales areas. Higgs (1986) favoured dextral strike-slip during late Westphalian to Stephanian times in the Bristol Channel region, as a possible mechanism to explain the northerly source of sediments in the Bude Formation of Cornwall and Devon. Thus, Cornubia would have been east of its present position during Westphalian A times. Holder & Leveridge (1986) suggested that Variscan dextral strike-slip took place in the British Channel, along a major fault that linked with the Bray Fault of northern France. Freshney & Taylor (1980) and Badham (1982) have also argued for Variscan strike-slip, and Max & Lefort (1982) have claimed that the Variscan Front in Ireland follows a dextral Shear zone. There is undoubtedly the need to carry out further work to substantiate the existence and disposition of any Variscan strike-slip in southern Britain and to resolve the problem of sediment supply to the Cornubian area. A number of major faults appear to have been active for a prolonged period during sedimentation in South Wales, particularly the Tawe (Swansea Valley) and Neath Disturbances (e.g. Owen 1971, 1974, 1984; Owen & Weaver 1983) which were probably founded on earlier Caledonian basement structures. Jones (1989) has also shown that contemporaneous activity of east-west folds influenced patterns of seam splitting. An easterly thinning of the Coal Measures of South Wales has been attributed to the prolonged activity of a positive axis in the Usk area (Squirrell & Downing 1964; Kelling 1974: Kellaway & Hancock 1983) which effectively acted as the eastern
margin to the South Wales Coalfield for much of the Westphalian. The Church Stretton Fault system, and its southerly continuation in South Wales, the Carreg Cennen Disturbance, is a structure that affected deposition throughout much of the Palaeozoic (e.g. Owen 1971), and appears to have influenced Westphalian sedimentation in both South Wales and the Welsh Borderland. The northern margin of the Westphalian A in the South Wales Coalfield probably lay just to the north of the present outcrop. Isopachytes of the thickness of Westphalian A from the Subcrenatum to the Vanderbeckei Marine Bands suggest thinning to the north, and the contemporaneously active Towy Anticline just north of the present outcrop has been suggested as a possible source of sediment (Owen 1964). A positive area of nondeposition or attenuated deposition in the Westphalian A has been suggested by several authors (e.g. Squirrell & Downing 1964; Owen 1971; Kelling 1974; Kellaway & Hancock 1983) which may extend from the line of the Usk Axis into the Severn Estuary, and this has been depicted as a land area on the map. The Forest of b e a n Coalfield is also shown as a nondepositional area. Sullivan (1964) claimed that a thin Westphalian A sequence was present here based on spores, but Cleal (1986a) has shown that this sequence is more likely to be Visran in age, on palaeobotanical grounds. Trueman (1947) and Wills (1956) suggested that the South Wales Coalfield may have been linked to that of the Midlands through the 'Hereford Straits'. Trueman (1947) claimed the presence of such a connection enabled the migration of non-marine bivalves between the Midlands and South Wales. The existence of over 1000 ft (300 m) of Westphalian A and B strata in the Clee Hills was cited as evidence for the Hereford Straits by Wills (1956). However the presence of these strata is equivocal in establishing the existence of the Hereford Straits, as the sequence contains extensive red beds of Etruria Marl facies and immature lithic sandstones indicating a marginal position similar to that occupied by the Wyre Forest area. Subsequent evidence has not been forthcoming, as the Westphalian is largely eroded away in the Welsh Borderland region, so that the existence of this feature will remain a matter of speculation. Westphalian A strata are locally unconformable on older rocks near the Wrekin, Shropshire, but are absent north of Shrewsbury, having possibly been eroded during late Westphalian times (Wills 1973). The northern limit of the Wales-London-Brabant High in North Wales and the Celtic Sea is largely conjectural, but is thought to lie to the south of the present outcrop limit. The Westphalian A of North Wales occurs above a thin Namurian sequence (Ramsbottom et al. 1978) and Westphalian A beds rest on Dinantian Limestones and Precambrian in Anglesey (Calver & Smith 1974), thus the boundary probably lies not far south of Anglesey. Between the coalfields of the Bristol area and the Kent Coalfield, the southern margin of the Wales-London-Brabant High is highly conjectural, as in this region, Westphalian A strata are absent at the surface and rare in the subsurface. In the Kent Coalfield, Westphalian A beds lie directly on Dinantian or Devonian. The northern boundary of the Wales-London-Brabant High can only be plotted approximately in East Anglia and the Southern North Sea Basin, but evidence is obtainable from offshore boreholes, especially in Quadrant 53. The boundary is similar to that suggested by Bless et al. (1977) for the northern limit of the Wales-London-Brabant Massif during the Westphalian C. Wills (1956) considered that there were many uncertainties regarding the nature and extent of this barrier, because of intra- and postWestphalian erosion, and many of these uncertainties have not yet been resolved. However, recent exploration by British Coal has shown that Westphalian A deposition took place further south in Warwickshire than originally proposed by Wills (Fulton & Williams 1988) and Westphalian A sedimentary and volcanic rocks have also been recently found in boreholes in South Oxfordshire and Berkshire (Foster et al 1989). Thus it is possible that Westphalian A deposition may have extended across the WalesLondon-Brabant High from the Midlands to Berkshire, although the Vanderbeckei Marine Band does not appear to have transgressed right across the high (Fulton & Williams 1988). Basic vulcanicity occurred during the Westphalian A in the East Midlands in Derbyshire, Nottinghamshire and Leicestershire (Francis et al. 1968; Burgess 1982; Kirton 1984) and also in the Oxfordshire-Berkshire region (Foster et al. 1989), and basic dykes and sills were emplaced elsewhere in the Midlands (Kirton 1984). Adjacent to the margins of the Wales-London-Brabant High, Westphalian A strata are generally unconformable on older rocks and sequences are considerably condensed. Thus, for instance, Westphalian A beds rest on Silurian in South Staffordshire (Ramsbottom et al. 1978), and Cambrian and Namurian in Warwickshire (Fulton & Williams 1988). Local primary red bed facies occur in close proximity to the margin of the Wales-LondonBrabant High, for example in South Staffordshire and the Wyre Forest (Ramsbottom et al. 1978; Besly 1983. 1988). In Northern England Fielding (1984a), Fielding & Johnson (1987) and Guion & Fielding (1988) showed that a number of faults, including the Nineth Fathom, Butterknowle and Morley--Campsall Faults were active during sedimentation. The Westphalian A palaeogeography of Scotland is .based mainly on the work of Kirk (1982. 1983, 1989). The major sediment source area for the Scottish Midland Valley was most likely Old Red Sandstone sediments in the Scottish Highlands and adjacent North Sea. At times, small source areas probably existed in the southwest Southern Uplands. Cheviots ~lnd Scottish Borders. Small upland areas may have existed in North Ayrshire and Canonbie in early Westphalian A times, which were gradually submerged as the depositional areas expanded. Volcanic activity extended from the
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
82
CARBONIFEROUS
Namurian into early Westphalian in North Ayrshire, and up to mid Westphalian B times in Fife and the Firth of Forth (Francis 1983). in Ayrshire, a thick Namurian Passage Group lava pile in the west formed a topographical high, that was progressively overstepped in the Westphalian A (Mykura 1967; Read 1988). Kirk (1983) and Read (1988), among others, have demonstrated that active tectonics accompanied by magmatism influenced Westphalian sedimentation in the Scottish Midland Valley, which Read (1988)considered to have been produced under a regime of dextral strike-slip. The Westphalian A of Arran is partially reddened, and has been interpreted by Kirk (1989) as the deposits of a proximal alluvial plain, draining a nearby source area to the north or northwest. A small outcrop of Westphalian occurs at Inninmore, Morvern (McGregor & Manson 1934; Love & Neves 1963), which is probably predominantly Westphalian B, but which may extend downwards into the Westphalian A (C. Cleal, pers. comm., 1985). The relationship of this outcrop to the main basins in the south is unclear, but it indicates that coal measures environments may have extended further north into the Scottish Highlands at certain times in the Westphalian. The Westphalian is restricted to a limited number of isolated outcrops in Ireland (Eagar 1975; Sevastopulo 1981b), although the distribution was probably much more extensive and has been stripped off by erosion. Thus the Westphalian palaeogeography of Ireland is extremely speculative. Westphalian A strata as young as the Modiolaris Zone have not been identified in Ireland, thus the map depicts the present distribution of older Communis zone deposits. The disposition of the Wales-London-Brabant High in Continental Europe is based mainly on Karrenburg (1971), Bless et al. (1977) and Ziegler (1982). The Westphalian A distribution and palaeogeography in the North Sea is somewhat speculative, but many wells are now being drilled into Carboniferous strata, particularly in the southern North Sea, which has considerably increased our understanding of this area (Eames 1975; Besly 1990; Leeder et al. 1990; Leeder & Hardman 1990). Haszeldine (1984) suggested that rifting may have commenced by Westphalian A-B times along the lines of the Rockall-Faeroe Trough to the west of the British Isles and along the Viking Graben in the northern North Sea. However, there is insufficient evidence available at present to show that these were definite palaeogeographical features in Early Westphalian times, so they have been omitted. Sedimentation also took place in a number of isolated basins within the Variscides in Europe during the Westphalian A. The Saar-Lorraine basin was formed in an intermontane depression between the Rheno-Hercynian and Saxo-Thuringian Massifs, and contains a thick Westphalian-Stephanian non-marine sequence, including Westphalian A beds at the base (Weingart 1976; Cleal 1984a). The approximate limits of the Saar-Lorraine basin are based on Karrenburg (1971) and Ziegler (1982). The Laval Basin, developed south of the North Armorican Shear Zone in Brittany, contains a sequence of Namurian shales with coal seams which extend up to the lower part of Westphalian A (Chauvel & Robardet 1979). These are intensely deformed and unconformably overlain by relatively-undeformed Stephanian sediments, indicating an episode of intra-Westphalian deformation. PDG
C9: Westphalian D A generalized palaeogeography is presented for the Westphalian D. Correlation of Westphalian D sequences in Britain is difficult, due to a lack of marine faunas, but Ramsbottom et al. (1978) is used as a basis. Cleal (1984a,b) has indicated that, using macrofloras, the base of the Westphalian D should be placed lower than that proposed by Ramsbottom et al. (1978), but it seems that accurate correlation of the base of the Westphalian D is not yet achievable in Britain. Thus, there is uncertainty regarding the age of many late Carboniferous deposits, both onshore and offshore (Besly 1988, 1990), and until the stratigraphy is refined there are problems of accurate sedimentological, tectonic and palaeogeographical synthesis. For this reason, the map depicts the present distribution of both Westphalian C and D deposits, which are often grouped together as the 'Barren Red Group' (Besly 1990), on account of their primary red colour in many areas. The map corresponds to Palstage 3a of Wills (1956) which falls approximately in the Tenuis Chronozone. In South Wales, this includes the upper part of the Pennant Measures, and in the Midlands the Halesowen and Newcastle Formations (the 'Upper Pennant-Halesowen Belt' of Wills 1956). Map C9 is somewhat conjectural, as there has been very little recent published work on these beds other than in South Wales, summarized by Kelling (1974). Recent work indicates that Westphalian D strata were probably deposited in alluvial channel and floodplain environments in Britain, with little marine influence (e.g. Besly 1988, 1990; Read 1988; Jones 1989). However, acritarchs, which are usually associated with marine or brackish influence, have recently been obtained from Westphalian D strata in Staffordshire (N. J. Turner, pers. comm. 1990). Westphalian D strata were deposited following a period of uplift and erosion, which Wills (1956) termed the 'First Malvernian or Symon Movements'. These resulted in the uplift and erosion of the Welsh and LondonBrabant Highs. By Westphalian D times the Variscan Mountain uplift had migrated northwards, such that the Cornubian Massif had become uplifted and was providing sediment to the north. Deformation and foreshortening continued until Stephanian times, when the Variscan Deformation Front reached its final position.
By mid Westphalian D times, the London-Brabant and Welsh Highs were reduced to a very subdued relief, and definite connections were established between South Wales and the Midlands (Wills 1956; Kellaway 1970). There is clear evidence of Westphalian D deposition in Oxfordshire and Berkshire where there is a thick coal-bearing sequence resting unconformably on Devonian or older strata (Poole 1969; Smith 1967; Foster et al. 1989). The "Hereford Straits' may have become fully established by Westphalian D times (Cleal 1987), allowing South Wales and the Midlands to be connected. Kelling (1974) and Jones (1989) have indicated that the Pennant sandstones of South Wales were derived from rising Variscan Mountains that lay not far to the south, probably in Devon and Cornwall. Stead (1974) also showed a southerly derivation of strata now shown to be mainly Westphalian D in age in the Bristol area. He recorded conglomerates in the most southerly areas, with lithic sandstones fining and thinning to the north. He envisaged rivers flowing north off Armorica across a broad, flat plain, which probably extended through Oxfordshire to Kent. Stead (1974) also recorded westerlydirected palaeocurrents in the Forest of Dean area, indicating a possible landmass in the Malverns, a view which was supported by Wills (1956, 1973). Thus it is envisaged that sandy lithic detritus was shed from the Variscan Mountains and travelled in a northerly direction through the Oxfordshire region (Besly 1988; Foster et al. 1989). This sandy detritus is represented by the sandstones of the Halesowen Formation, which occurs near to the base of the Westphalian D in the Midlands. However, this southerly derived lithic detritus does not appear to have been transported further north than Warwickshire, South Staffordshire and Nottinghamshire. The Keele Formation, which overlies the Halesowen Formation in the Midlands, together with its lateral equivalents, generally consists of mudstones with some sandstones and thin Spirorbis limestones. The sandstones are petrographically different from those of the Halesowen Formation and this, together with limited palaeocurrent evidence, indicates that they may have been of northerly derivation (Besly 1988, 1990). The margins of the London-Brabant and Welsh Highs are largely speculative, and it is possible that the landmasses were much smaller at times than depicted. The position of the southern boundary of the London-Brabant High is particularly uncertain, as the Westphalian D may have suffered erosion prior to deposition of the younger Permian and Mesozoic strata. However, spores of late Westphalian B to D age have been identified from a sequence of mudstones and sandstones at Middleton I Borehole, near Portsmouth. Westphalian D strata have been identified in the Somerton Borehole, near Norwich, and Allsop (1984) and Allsop & Jones (1981) suggested an area of Westphalian, northeast of Norwich, based on geophysical evidence. There are indications of the positive influence of the landmasses of the Midlands in Upper Westphalian times. An unconformity occurs at or near the base of the Westphalian D adjacent to the landmasses (the 'Symon Unconformity'), and the Westphalian D is unconformable on earlier Westphalian or even pre-Carboniferous beds in the Midlands. This unconformity has also been recognized in several offshore wells (e.g. 53/10-1, 53/12-3) and on seismic sections shot adjacent to the margins of the London-Brabant High (Tubb et al. 1986). Thus Westphalian D strata appear to overlap onto the London-Brabant High that underwent periods of uplift and erosion from mid-Westphalian B to early Westphalian D, recorded by the spread of locally-derived red beds (Besly 1983, 1988). According to recent evidence of Cleal (1986b), the Forest of Dean, Severn and Newent areas continued as positive areas throughout much of the Westphalian, and did not begin to receive sediment until Upper Westphalian D times. The Westphalian D is generally thin in Lancashire, Yorkshire and Nottinghamshire, having suffered pre-Permian erosion, thus it is difficult to form a complete picture of Westphalian D sedimentation in these areas. Wills (1956, p 76) conjectured that uplift may have commenced in the Pennine area, but evidence is inconclusive. It is possible that landmasses were emergent in Northern England by Westphalian D times (e.g. in parts of Cumbria and Northumberland-Durham), but the lack of detailed studies causes problems of interpretation. In the Midland Valley of Scotland, reddened sediments in the Upper part of the Westphalian are difficult to date as fossils are scarce and strata preservation poor, but Westphalian D strata have been identified (Mykura 1967). An alluvial environment of deposition is likely for these, and marine beds are lacking. At present no palaeocurrent data are available. Westphalian D strata are virtually absent in Ireland, probably having suffered pre-Permian erosion. However, Westphalian D beds have been indicated offshore of Ireland in Well 13/3-1 north of Ireland, and 33/22-1 in the Kish Bank Basin (Jenner 1981). Westphalian D deposits have also been encountered in wells to the west of Ireland (Tate & Dobson 1989), which have been compared with the late Westphalian of the English Midlands. Tate & Dobson (1989) have described Westphalian D-Stephanian limestones with marine faunas in well 34/15-1 in the Porcupine Basin, but the significance of this still needs to be assessed. Clayton et al. (1986) identified Westphalian D rocks onshore in Ireland in mineral exploration borehole W80.5 in Wexford and argued that late Westphalian deposition may have extended across the Irish Sea towards Anglesey. The Westphalian D rocks contain an assemblage of recycled spores of Devonian to Westphalian A age. It is suggested that these were derived from the erosion of rising Variscan structures that were developing south of the Variscan Front.
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
84
CARBONIFEROUS
Westphalian D strata have been recognized in a number of North Sea wells (Besly 1990; Leeder & H a r d m a n 1990) and have been identified in Continental Europe, north of the Variscan Deformation Front (Bless et al. 1977; Thorez & Bless 1977). The Ravenspurn Anticline and the Murdoch Anticline identified by Leeder & Hardman (1990) in the southern North Sea Basin were interpreted as growth folds that acted as local sediment sources in the Westphalian C/D. The Mid-Netherland High is a feature that came into prominence during the Lower Westphalian C (Bless et al. 1977) and may have continued until Westphalian D times as a subdued feature. Shallow Marine Westphalian C/D rocks have been described from the Oslo Graben (Olausson, 1981). It was suggested by Haszeldine (1984) that the marine faunas of the Oslo region may have exploited a Boreal connection through a proto-Viking Graben. However, Leeder (1988) cast doubt on this possibility, and speculated on derivation via the Uralian Sea. Due to uncertainties about the affinities of the faunas, and the controversy regarding the nature of the Oslo Graben, the proto-Viking Graben is not shown on the map. A number of Westphalian limnic basins have been preserved within the Variscan fold belt. The Saar-Lorraine Basin contains a thick, limnic Westphalian D sequence (Kneuper 1971; Bless et al. 1977; Cleal 1984a), and the Plessis Basin of Brittany, which contains Westphalian B and C beds may also contain Westphalian D deposits (Bless et al. 1977). PDG Assistance with this compilation was given by B. M. Besly, C. J. Cleal, E. H. Francis, R. Higgs, S. Monro, S. C. Nolan, N. J. Riley, E. B. Selwood, P. Wallace and G. M. Walkden. P. D. G. acknowledges other assistance from I. Fulton, R. Goossens, M. Holder, D. Ince, A. Jones, M. Kirk, A. Matthews and A. H. V. Smith; and Oxford Polytechnic for secretarial and drafting services.
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References References not listed here are cited in George et al. (1976) or Ramsbottom et al. (1978). ALLSOP, J. M. 1984. Geophysical appraisal of a Carboniferous basin in north-east Norfolk, England. Proceedings of the Geologists" Association, 95, 175-180. & JONES, C. M. 1981. A pre-Permian palaeogeological map of the East Midlands and East Anglia. Transactions of the Leicester Literary and Philosophical Society, 75, 28-33. BADHAM, J. P. N. 1982. Strike slip orogens--an explanation for the Hercynides. Journal of the Geological Society, London, 139, 493-504. BARNES, R. P. & ANDREW,J. R. 1986. Upper Palaeozoic ophiolite generation and obduction in North Cornwall. Journal of the Geological Society, London, 143, 117-124. BARRETT, P. A. 1988. Early Carboniferous of the Solway Basin: a tectono-stratigraphic model and its bearing on hydrocarbon potential. Marine and Petroleum Geology, 5, 271-281. BATSTONE,A. E. 1959. The structure and tectonic history of the Tintagel-Davidstow area. Transactions of the Royal Geological Society of Cornwall, 19, 17-32. BESLY, B. M. 1983. The Sedimentology and Stratigraphy of Red Beds in the Westphalian A to C of Central England. PhD Thesis, University of Keele. 1987. Sedimentological evidence for Carboniferous and Early Permian palaeoclimate of Europe. Annales de la Socibtb Gbologique du Nord, 106, 131-143. 1988. Palaeogeographic implications of late Westphalian to early Permian redbeds, Central England. In: BESLY,B. M. & KELLING,G. (eds) Sedimentation in a Synorogenic Basin Complex: the Upper Carboniferous of Northwest Europe. Blackie, Glasgow, 200-221. -1990. Carboniferous. In: GLENNIE, K. W. (ed.) Introduction to the Petroleum Geology of the North Sea. Blackwell, Oxford, 90-119. BLESS, M. J. M., BOUCKAERT,J., CALVER, M. A., GRAULICH,J. M. & PAPROTH,E. 1977. Palaeogeography of Upper Westphalian deposits in N.W. Europe with reference to the Westphalian C north of the Mobile Variscan Belt. Mededelingen Rijks Geologische Dienst N.S., 28, 101-127. BLUCK, B. J. 1978. Sedimentation in a late orogenic basin: the Old Red Sandstone of the Midland Valley of Scotland. In: BOWLS,D. R. & LEAKE,B. E. (eds) Crustal evolution in northern Britain and adjacent areas. Special Issue 10, Geological Society of London. 1980. Evolution of a strike-slip fault-controlled basin, Upper Old Red Sandstone, Scotland. In: BALLANCE, P. F. & READING, H. G. (eds.) Sedimentation in oblique-slip mobile zones. International Association of Sedimentologists. Special Publication, 4. BROWNE, M. A. E. 1980. Stratigraphy of the lower Calciferous Sandstone Measures in Fife. Scottish Journal of Geology, 6, 321-328. BURGESS,I. C. 1982. The stratigraphical distribution of Westphalian volcanic rocks to the east and south of Nottingham. Proceedings of the Yorkshire Geological Society, 44, 29-44. CAMERON, I. B. & STEPHENSON,D. 1985. The Midland Valley of Scotland, British Regional Geology, 3rd edn, British Geological Survey. CHANDLER, P & ISAAC, K. P. 1982. The geological setting, geochemistry, and significance of lower Carboniferous basic volcanic rocks in central south-west England. Proceedings of the Ussher Society, 5, 279-288. CHARSLEY, T. J. 1983. Early Carboniferous rocks of the Swinden No. 1 Borehole, west of Skipton, Yorkshire. Report of the British Geological Survey, 16, No. 84/1, pp. 5-12. CHAUVEL,J-J. & ROBARDET,M. 1979. France: Introduction ~i la g6ologie de l'ouest. Twenty-sixth International Geological Congress Guide Book. Bulletin de la Soeidtb Gbologique et Mindralogique de Bretagne, 11, 1-191. CHISHOLM, J. 1. & DEAN, J. i . 1974. The Upper Old Red Sandstone of Fife and Kinross: a fluviatile sequence with evidence of marine incursion. Scottish Journal of Geology, 10, 1-30. CLAYTON,G., SEVASTOPULO,G. D. & SLEEMAN,A. G. 1986. Carboniferous (Dinantian and Silesian) and Permo-Triassic rocks in south County Wexford, Ireland. Geological Journal, 21,355-374. CLEAL,C. J. 1984a. The Westphalian D floral biostratigraphy of Saarland (Fed. Rep. Germany) and a comparison with that of South Wales. Geological Journal, 19, 327-351. 1984b. The recognition of the Westphalian D Stage in Britain. Geological Maga-ine, 121. 125-129. -
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1986a. Plant macrofossils from the Edgehills Sandstone, Forest of Dean. Bulletin of the British Museum, Natural History (Geology), 40, 235-246. 1986b. Fossil plants of the Severn Coalfield and their biostratigraphical significance. Geological Maga'.me, 123, 553-568. - - 1987. Macrofloral biostratigraphy of the Newent Coalfield, GIoucestershire, Geological Journal, 22, 207-217. CRAIG, G. Y. (ed.) 1983. The geology of Scotland, 2rid edition, Scottish Academic Press, Edinburgh. DAVIES,J. S., RILEY, N. J. & WILSON,D. 1989. Distribution of Chadian and earliest Arundian strata in north-east Wales: Implications for Dinantian (Carboniferous) lithostratigraphy and palaeogeography. Geological Journal, 24, 3 i-48. DEEGAN,C. E. 1973. Tectonic control of sedimentation at the margin of a Carboniferous depositional basin in Kircudbrightshire. Scottish Journal of Geology, 9, 220-276. DEWEY,J. F. 1982. Plate tectonics and the evolution of the British Isles. Journal of the Geological Society, London, 139, 371~,1-2. EDMONDS, E. A., WHITTAKER,A. & WILLIAMS,B. J. 1985. Geology of the country around Ilfracombe and Barnstaple. Memoirs of the Geological Survey of Great Britain. - - , WILLIAMS,B. J. & TAYLOR,R. T. 1979. Geology of Bideford and Lundy Island. Memoir of the Geological Survey of Great Britain. ELLIOTT, T. 1976. Carboniferous sedimentary cycles produced by river-dominated elongate deltas. Journal of the Geological Society, London, 132, 199-208. FIELDING,C. R. 1984a. A coal depositional model for the Durham Coal Measures of N.E. England. Journal of the Geological Society, London, 141, 919-931. -1984b. Upper delta-plain lacustrine and fluviolacustrine facies from the Westphalian of the Durham Coalfield. Sedimentology, 31, 547-567. 1986. Fluvial channel and overbank deposits from the Westphalian of the Durham Coalfield, N.E. England. Sedimentology, 33, 119-140. - - & JOHNSON,G. A. U 1987. Sedimentary structures associated with extensional fault movement from the Westphalian of N.E. England. In: COWARD, M. P., DEWEY, J. F. & HANCOCK, P. L. (eds) Continental Extensional Tectonics. Geological Society, London, Special Publication, 28, 511-516. FOSTER, D., HOLLIDAY,D. W., JONES, C. M., OWENS, B. & WELSH, A. 1989. The concealed Upper Palaeozoic rocks of Berkshire and South Oxfordshire. Proceedings of the Geologists" Association, 100, 395-407. FRANCIS,E. H. 1983. Carboniferous-Permian Igneous Rocks. In: CRAIG,G. Y. (ed.) Geology of Scotland 2rid edition, Scottish Academic Press, Edinburgh, 297-324. 1991. Carboniferous-Permian igneous rocks, In: CRAIG, G. Y. (ed.) The Geology of Scotland, 3rd edition, Scottish Academic Press, Edinburgh. - - , SMART,J. G. D. & RAISBECK,D. E. 1968. Westphalian volcanism at the horizon of the Black Rake in Derbyshire and Nottinghamshire. Proceedings of the Yorkshire Geological Society, 36, 395--416. FRESHNEV, MCKEOWN,M. C. & WILLIAMS,M. 1982. Geology of the coast between Tintagel and Bude. Memoirs of the Geological Survey of Great Britain. -& TAYLOR, R. 1980. The Variscides of South-West Britain. Twenty-sixth International Geological Congress, Excursion Guidebook I, 379-387. FULTON, I. M. & WILLIAMS,H. 1988. Palaeogeographical change and controls on Namurian and Westphalian A/B sedimentation at the southern margin of the Pennine Basin, Central England. In: BESLY, B. M. & KELL!NG, G. (eds) Sedimentation in a Synorogenic Basin Complex--the Upper Carboniferous of Northwest Europe. Blackie, Glasgow, 153-177. GAWTHORPE, R. L., GUTTERIDGE, P. & LEEDER, M. R. 1989. Late Devonian and Dinantian basin evolution in northern England and north Wales, In: ARTHURTON,R. S., GUrTERIDGE,P. & NOLAN,S. C. (eds) The role of tectonics in Devonian and Carboniferous sedimentation. Yorkshire Geological Society, Occasional Publication, 6, 1-23. GEORGE, G. T. 1970. The sedimentation of Namurian sequences in South Pembrokeshire. PhD thesis, University of Wales. GEORGE,T. N. 1974. Lower Carboniferous rocks in Wales, In: OWEN,T. R. (ed.) The Upper Palaeozoic and post-Palaeozoic rocks of Wales, University of Wales Press, Cardiff, 85-115. , JOHNSON, G. A. L., MITCHELL,M., PRENTICE,J. E., RAMSBOTTOM,W. H. C., SEVASTOPULO,G. D. & WILSON,R. B. 1976. A correlation of the Dinantian rocks in the British Isles. Geological Society, London, Special Report, 7. GILLIGAN,A. 1920. The petrography of the Millstone Grit of Yorkshire. Quarterly Journal of the Geological Society of London, 75, 251-294. GUION, P. D. 1978. Sedimentation of Interseam Strata and Some Relationships with Coal Seams in the East Midlands Coalfield. PhD thesis, Council of National Academic Awards. 1984. Crevasse splay deposits and roof-rock quality in the Threequarters Seam (Carboniferous) in the East Midlands Coalfield, U.K. In: RAHMANI,R. A. & FLORES, R. M. (eds) Sedimentology of Coal and Coal-Bearing Strata. Special Publications of the International Association of Sedimentologists, 7, 291-308. 1987. Palaeochannels in mine workings in the High Hazles Coal (Westphalian B), Nottinghamshire Coalfield, England. Journal of the Geological Socieo', London, 144, 471-488. & FIELDING,C. R. 1988. Westphalian A and B sedimentation in the Pennine Basin, UK. In: BESLY,B. M. & KELLING,G. (eds) Sedimentation in a Synorogenic Basin Complex." the Upper Carboniferous of Northwest Europe, Blackie, Glasgow, 153-177. HASZELDINE, R. S. 1984. Carboniferous North Atlantic palaeogeography: stratigraphic evidence for rifting, not megashear or subduction. Geological Magazine, 121,443-463. HEWARD, A. 1976. Sedimentation Patterns in Coal-Bearing Strata, Northern Spain and Great Britain. DPhil thesis, University of Oxford. H1GGINS,A. C. & VARKER,W. J. 1982. Lower Carboniferous conodont faunas from Ravenstonedale, Cumbria. Palaeontology, 25, 145-166. HIGGS, K., CLAYTON, G. & KEEGAN, J. B. 1988. Stratigraphic and systematic palynology of the Tournaisian rocks of Ireland. Geological Survo' of Ireland, Special Paper, 7. HIGGS, R. 1984. Possible wave-influenced sedimentary structures in the Bude Formation (Lower Westphalian, South-West England), and their environmental implications. Proceedings of the Ussher Society, 6, 88-94. -1986. A Facies Analysis of the Bude Formation (Lower Westphalian), S.W. England. DPhil Thesis, University of Oxford. HITCrlEN, K. & RITCHIE,J. D. 1987. Geological review of the West Shetland area, In: BROOKS, J. & GLENNIE,K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 737-749. HOLDER, M. T. & LEVERIDGE,B. E. 1986a. A model for the tectonic evolution of south Cornwall. Journal t~f the Geological Society, London, 143, 125-134. & LEVERIDGE, B. E. 1986b. Correlation of the Rhenohercynian Variscides. Journal t~f the Geological Socie t l', London, 143, 141- 147. -
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Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
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HOLDSWORTH, B. K. 1966. A preliminary study of the palaeontology and paiReDenvironment of some Namurian limestone "bullions". Mercian Geologist, 1, 315-337. HOUSE, M. R., RICHARDSON,J. B., CHALONER,W. G., ALLEN,J. R. L., HOLLAND, C. H. t~ WESTOLL,T. S. 1977. A correlation of Devonian rocks in the British Isles. Geological Society, London. Special Report, 8. JENNER, J. K. 1981. The structure and stratigraphy of the Kish Bank Basin. In: ILL1NG, L. V. 8t. HOBSON,G. D. (eds) Petroleum Geology of the Continental Shelf of North- West Europe. Heyden, London, 426~.3 I. JONES, G. LI., SOMERVILLE, I. D. ~r STROGEN, P. 1988. The Lower Carboniferous (Dinantian) of the Swords area: Sedimentation and tectonics of the Dublin Basin. Ireland. Geological Journal, 23, 221-248. JONES, J.' 1989. The influence of contemporaneous tectonic activity on Westphalian sedimentation in the South Wales Coalfield. In: ARTHURTON, R. S., GUTTERtDGE, P. & NOLAN, S. E. (eds) The Role of Tectonics in Devonian and Carboniferous Sedimentation in the British Isles. Yorkshire Geological Society Occasional Publication, 6, 243-253. KARRENBURG,H. 1971. Introduction. In: the Carboniferous Deposits in the Federal Republic of Germany--a Review (English Translation). Fortschritte in der Geologic yon Rheinland und Westfalln, 19, 7-10. KELLAWAY, G. A. & HANCOCK, P. L. 1983. Structure of the Bristol District, the Forest of Dean and the Malvern Fault zone. In: HANCOCK, P. L. (ed.) The Variscan Fold Belt in the British Isles. Adam Hilger, Bristol, 88-107. KELLING,G. 1974. Upper Carboniferous sedimentation in South Wales. In: OWEN, T. R. (ed.) The .Upper Palaeozoic and Post-Palaeozoic Rocks of Walls~ University of Wales Press, Cardiff, 185-224. KIMBER, R. 1984. Clastic deposition beneath the lowermost Carboniferous limestones of north-west England, European Dinantian Environments 1st meeting, Manchester, Abstracts, Department of Earth Sciences, Open University. KIRK, M. 1982. Palaeoenvironments and geography in the Westphalian A and B Coalfields of Scotland. Open Earth, 17, 536-537. -1983. Sedimentology and Palaeogeography in the Westphalian A and B Coalfields of Scotland. PhD thesis, University of Strathclyde. 1989. Westphalian alluvial plain sedimentation, Isle of Arran, Scotland. Geological Magazine, 126, 407-421. KIRTON, S. R. 1984. Carboniferous volcanicity in England with special reference to the Westphalian of the East and West Midlands. Journal of the Geological Society, London, 141, 160-170. KNEUPER, G. 1971. 1) The Saar-Nahe District a) Delimitation and geological history. In: The Carboniferous Deposits in the Federal Republic of Germany--A Review (English Translation). Fortschritte in der Geologie yon Rheinland und Westfalln, 19, 146-151. LEE, A. G. 1988. Carboniferous basin configuration of Central and Northern England modelled using gravity data. In: BESLY, B. M. & KELLING, G. (eds) -
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Sedimentation in a Synorogenic Basin Complex. the Upper Carboniferous of Northwest Europe. Blackie, Glasgow, 69-84. LEEDER, M. R. 1974. Lower Border Group (Tournaisian) fluvio-deltaic sedimentation and palaeogeography of the Northumberland Basin. Proceedings of the Yorkshire Geological Society, 40, 129-180. 1982. Upper Palaeozoic basins of the British Isles--Caledonide inheritance versus Hercynian plate margin processes. Journal of the Geological Society, London, 139, 479--491. -1987. Tectonic and palaeogeographic models for lower Carboniferous Europe, In: MILLER, J., ADAMS, A. E. & WRIGHT, V.P. (eds)European Dinantian Environments. Wiley, Chichester, 1-20. 1988. Recent developments in Carboniferous geology: a critical review with implications for the British Isles and N.W. Europe. Proceedings of the Geologists' Association, 99, 73-100. -& HARDMAN,M. 1990. Carboniferous geology of the southern North Sea Basin and controls on hydrocarbon prospectivity. In: HARDMAN,R. F. P. & BROOKS,J. (eds) Tectonic' Events Responsible for Britain "s Oil and Gas Reserves. Geological Society, London, Special Publication, 55, 87-104. AL-BIATTY,H., MCMAHON, A. & HARDMAN,M. 1990. Carboniferous stratigraphy, sedimentation and correlation of well 48/3-3 in the southern North Sea Basin: integrated use of palynology, natural gamma/sonic logs and carbon/sulphur geochemistry. Journal of the Geological Society, London, 147, 287-300. LEES, A., HALLET,V. & I-[1BO,D. 1985. Facies variation in carbonate buildups. Part I: A model from Belgium. Geological Journal, 20, 133-158. & MILLER, J. 1985. Facies variation in carbonate buildups from Africa and north America. Geological Journal, 20, 159-180. LOVE, L. G. & NEVES, R. 1963. Palynological evidence on the age of the Carboniferous at lnninmore. Transactions of the Geological Society of Glasgow, 25, 61-71. MACGREGOR, M. & MANSON, W. 1934. The Carboniferous Rocks of Inninmore, Morvern. Summary of Progress. Geological Survey of Great Britain for t933. Pt. II, 74-84. MAX, M. D. & LEFORT, J. P. 1984. Does the Variscan Front in Ireland follow a dextral shear zone? In: HUTTON, D. H. W. & SANDERSON,D. J. (eds) Variscan Tectonics of the North Atlantic Region. Geological Society, London, Special Publication, 14, 177-183. MEADOWS, N. S., MACCHI, L., CUBITT, J. M. & JOHNSON, B. 1987. Sedimentology and reservoir potential in the west of Shetland, UK, exploration area. In: BROOKS, J. & GLENNIE, K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 723-736. MELVIN, J. 1986. Upper Carboniferous fine-grained turbiditic sandstones from Southwest England: a model for growth in an ancient delta-fed subsea fan. Journal of Sedimentary Petrology, 56, 19-34. MITCHELL, M. & REYNOLDS, M. J. 1981. Early Tournaisian rocks at Lilleshall, Shropshire. Geological Magazine, 118, 699-702. MYKURA, W. 1967. The Upper Carboniferous rocks of South West Ayrshire. Bulletin of the Geological Survey of Great Britain, 26, 23-98. OGUIKE, R. O. 1969. Sedimentation of the Middle Shales (Upper Namurian ) of the South Wales Coalfield. PhD Thesis, University of Wales. OLAUSSEN,S. 1981. Marine incursion in Upper Palaeozoic sedimentary rocks of the Oslo Graben region, Southern Norway. Geological Magazine, 118, 281-288. OWEN, T. R. 1964. The tectonic framework of Carboniferous sedimentation in South Wales. In: VAN STRAATEN, L. M. J. U. (ed.) Deltaic and Shallow Marine Deposits. Developments in Sedimentology, 1. Elsevier, Amsterdam, 301-307. -1971. The relationship of Carboniferous sedimentation to structure in South Wales. Sixikme Congrds International de Stratigraphie et de G~ologie du Carbonifkre, Sheffield 1967, 3, 1305-1315. -
-
-
-
,
-
-
R A I S W E L L ,
R . ,
---
1974. The Variscan orogeny in Wales. In: OWEN, T. R. (ed.) The Upper Palaeozoic and Post-Palaeozoic Rocks of Wales. University of Wales Press, Cardiff, 285-294. 1984. Upper Palaeozoic Wales--a review of studies in the past 25 years.
Proceedings of the Geologists' Association, 95, 349-364. & WEAVER,J. D. 1983. The structure of the main South Wales Coalfield and its margins. In: HANCOCK, P. L. (ed.) The Variscan FoM Belt in the British Isles. Adam Hilger, Bristol, 74-87. PEACOCK, J. D., BERRIDGE,N. G., HARRIS,A. L. & MAY, F. 1968. The geology of the Elgin district. Memoir of the Geological Survey of Great Britain. RAMSAY,A. T. S. 1989. Tectonics and sedimentation of the late Dinantian limestones in South Wales. In: ARTHURTON,R. S., GUTTERIDGE,P. & NOLAN, S. C. (eds) The roll of tectonics in Devonian and Carboniferous sedimentation. Yorkshire Geological Society, Occasional Publication, 6, 225-241. RAMSBOTTOM,W. H. C. 1979. Rates of transgression and regression in the Carboniferous of N.W. Europe. Journal of the Geological Society, London, 136, 147153. - - , CALVER,M. A., EAGER, R. M. C., HODSON, F., HOLL1DAY,D. W., STUBBLEEIELD, C. J. & WILSON, R. B. 1978. A correlation of Silesian rocks in the British Isles. Geological Society, London, Special Report, 10. READ, W. A. 1988. Controls on Silesian sedimentation in the Midland Valley of Scotland. In: BESLEV,B. M. & KELLING,G. (eds) Sedimentation in a Synorogenic Basin Complex: the Upper Carboniferous of Northwest Europe. Blackie, Glasgow, 222-241. RIDD, M. F. 1981. Petroleum geology west of the Shetlands. In: ILLING, L. V. & HOBSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North- West Europe, Heyden, London, 414-425. RIDER, M. H. 1969. Sedimentological studies in the West Clare Namurian, Ireland, and the Mississippi River delta. Thesis, University of London. RILEY, N. J., VARKER, W. J., OWENS, B., HIGGINS, A. C. & RAMSBOTTOM,W. H. C. 1987. Stonehead Beck, Cowling, North Yorkshire, England: a British proposal for the mid-Carboniferous boundary stratotype. Courier Forschungsinstitut Senckenberg, 98, 149-177. ROGERS, D. A., MARSHALL, J. E. A. & ASTIN, T. R. 1989. Devonian and later movements on the Great Glen fault system, Scotland. Journal of the Geological Society, London, 146, 369-372. SCOTESE, C. R., BAMBACH,R. K., BARTON, C., VAN DER VOO, R. d~ ZIEGLER, A. M. 1979. Palaeozoic base maps. Journal of Geology, 87, 217-277. SEVASTOPULO, G. D. 1981a. Lower Carboniferous, In: HOLLAND, C. H. (ed.) A Geology of Ireland, Scottish Academic Press, Edinburgh, 147-172. -1981b. Upper Carboniferous. In: HOLLAND, C. H. (ed.) A Geology of Ireland. Scottish Academic Press, Edinburgh, 173-187. SMITH, A. G., HURLEY, A. M. ~. BRIDEN, J. C. 1981. Phanerozoic Palaeocontinental World Maps. Cambridge University Press. SMITH, A. H. V. 1987. Stratigraphic ranges of selected miospores in coal seams of Upper Coal Measures age in Oxfordshire and S.C. Warwickshire. Journal of Micropalaeontology, 6, 21-37. SMITH,N. J. P., 1985. Map 1. Pre-Permian Geology of the United Kingdom (South). Scale 1 : 1,000,000. British Geological Survey, Keyworth. SOMERVILLE,I., STRANK,g. R. E. ~g WELCH, A. 1989. Chadian faunas and flora from Dyserth; Depositional environment and palaeogeographic setting of Vis6an strata in northeast Wales. Geological Journal, 24, 494i6. SQUIRRELL,H. C. & DOWNING, R. A. 1964. The attenuation of the Coal Measures in the south-east part of the South Wales Coalfield. Bulletin of the Geological Survey of Great Britain, 21, 119-132. STEAD, J. T. G. 1974. The Sedimentology of the Upper Coal Measures of the Forest of Dean and Adjacent Areas. PhD thesis, University of Wales. STRANK, A. R. E. 1987. The stratigraphy and structure of Dinantian strata in the east Midlands, UK. In: MILLER, J., ADAMS, A. E. & WRIGHT, V. P. (eds) European Dinantian Environments, Wiley, Chichester, 157-176. TATE, M. P. & DOBSON,M. R. 1989. Pre-Mesozoic geology of the western and northwestern Irish continental shelf. Journal of the Geological Society, London, 146, 229-240. THOMAS, J. M. 1988 Basin history of the Culm Trough of Southwest England. In: BESLY, B. M. & KELLING, G. (eds) Sedimentation in a Synorogenic Basin Complex: the Upper Carboniferous of Northwest Europe. Blackie, Glasgow, 2437. THOREZ, J. & BLESS, M. J. M. 1977. On the possible origin of the Lower Westphalian D Neeroeteren Sandstone (Campine, Belgium). Mededelingen Rijks Geologische Dienst. N,S. 28, 128-132. TRUEMAN, A. E. 1947. Stratigraphical problems in the Coal Measures of Great Britain. Quarterly Journal of the Geological Society of London, 103, Ixv--civ. Tua8, S. R., SOULSBY, A. & LAWRENCE, S. R. 1986. Palaeozoic prospects on the north flank of the London--Brabant massif: In: BROOKS,J., GOFF, J. C. & VAN HOORN, B. (eds) Habitat of Palaeozoic gas in N.W. Europe. Geological Society, London, Special Publication, 23, 55--72. WALKDEN, G. M. 1987. Sedimentation and diagenetic style in late Dinantian carbonates of Britain. In: MILLER,J., ADAMS, A. E. & WRIGHT, V. P. (eds) European Dinantian Environments, Wiley, Chichester. 131-156. WEINGART, H. W. 1976. Das Oberkarbon in der Tiefbohrung Saar 1. Geologisches Jahrbuch, A27, 399-408. WHITELEV, M. J. 1984. Shallow water Dinantian sediments in south-east Cornwall. Proceedings of the Ussher Society, 6, 137-141. WHITTAKER,A. & SCRIVENER,R. C. 1982. The geology of the Institute of Geological Sciences deep borehole (Devonian-Carboniferous) at Knap Farm, Cannington Park, Somerset. Report of the Institute of Geological Sciences, 82/5. WILLS,L. J. 1948. The Palaeogeography of the Midlands. Liverpool University Press. -1951. A Palaeogeographical Atlas of the British Isles and Adjacent Parts of Europe. Blackie, London. -1956. Concealed Coalfields, Blackie, London. -1973. A Palaeogeological Map of the Palaeozoic Floor Below Permian and Meso:oic Formations in England and Wales. Geological Society, London, Memoir, 7. WILSON, D., DAVIES,J. R., SMITH, M. • WATERS, R. A. 1988. Structural controls on Upper Palaeozoic sedimentation in south-east Wales. Journal of the Geological Society, London. 145~ 901-914. ZIEGLER, P. A. 1982. Geological Atlas of Western and Central Europe. Shell lnternationale Petroleum Maatschappij B.V., Elsevier, Amsterdam. 1984. Caledonian and Hercynian crustal consolidation of Western Europe--a working hypothesis. Geologie en Mijnbouw, 63, 93-108. -
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Permian D. B. S M I T H
& J . C. M . T A Y L O R
with contributions from R. S. Arthurton, M. E. Brookfleld & K. W. Glennie
Permian strata in the British Isles crop out mainly in northern and central England but are extensivein the subsurface both on land and in several adjoining offshore areas. Their base is defined as in Smith et al. (1974) and their top is within red beds, overlying the Zechstein evaporites. We emphasize that both the base and the top of the nominally Permian rocks lie in continental strata almost devoid of stratigraphically useful fossils and that, accordingly, these boundaries are only doubtfully correlated with internationally acceptable biostratigraphic standards. Subdivision of British Permian strata into Lower and Upper series follows the traditional view summarized by Smith et al. (1974). The junction between the series is taken at the incoming of marine strata in northern England and adjoining offshore areas, and at approximately equivalent levels in continuous continental sequences elsewhere; recent limited palynological studies suggest that the early Permian-late Permian transition adopted here and in most of northwest Europe may be mid or late Kazanian or even Tatarian in age which is somewhat younger than the base-Kazanian/ Ufimian position taken in more continuous marine sequences. DBS
PI: Early Permian Fhere has been no comprehensive revision of the stratigraphy and nomenclature of early Permian strata in and around the British Isles since the work of Smith et al. (1974) and Rhys (1974), but these aspects of the early Permian continental deposits of several cuvettes and inland drainage basins in southwest Scotland were reviewed by Brookfield (1978) and the age of continental deposits in the Elgin area was reconsidered by Glennie & Buller (1983) and Benton & Walker (1986); their findings are incorporated here. Strata of the East Irish Sea and adjoining basins have been reviewed by Jackson and others (1987) and the nomenclature of late Permian marine and other strata in northeast England has been extensively revised by Smith et al. (1986).
Early early P e r m i a n The first part of the early Permian in the British Isles and surrounding areas was dominated by desert erosion and deposition; during this time, the area lay deep within the Pangaean supercontinent, in the belt of easterly trade winds and within a few degrees of the equator. The early Permian scene was set by the steady northwards drift from the equatorial rain forest belt of the Westphalian and by the profound geographical, structural and climatic changes resulting from Variscan faulting, folding and uplift; these changes included the expulsion of all seas and the creation of a diverse landscape which owed much of its character to the underlying rocks; thus, for example, the extensive tracts of newly uplifted Carboniferous strata probably gave rise to juvenile tabular landscapes of receding scarps and cuestas, whereas pre-Carboniferous rocks probably gave rise mainly to less-ordered immature country with deep valleys and a local relief of several hundreds of metres. Volcanoes and their products dominated the early Permian scenery in much of the Midland Valley of Scotland, in eastern Ulster, in the Exeter area of southwest England and in parts of the North Sea including the flanks of the Central Graben. Western parts of the British Isles area during these earliest early Permian times probably looked very similar to parts of the basin-and-range province of southwestern USA, with the topography inherited from the patchy Permo-Carboniferous volcanism gradually declining with time; little can be inferred about the palaeogeography of eastern areas at this time. It is possible that Variscan uplift and movement along some faults continued well into early Permian time, but this is difficult to prove on the evidence now preserved. Perhaps more importantly, tension, possibly associated with the onset of rifting in the North Atlantic area (Russell 1976; Russell & Smythe 1978), led to the creation of a number of isolated and interconnected subsiding troughs and trap-door (half-graben) basins in western Scotland, western England, North Wales, the Celtic Sea, South West Approaches and in central and eastern parts of the North Sea; thick continental sediments accumulated in the larger of these troughs and basins. Related tension in the northern and southern North Sea areas and in the eastern Irish Sea led to more uniform general crustal thinning and subsidence but erosion remained dominant and few deposits of early early Permian age have been preserved there (Glennie 1984); more complex patterns of subsidence, probably including local structural inversion, has 9 1992The GeologicalSociety
been inferred by Kent (1980), Glennie & Boegner (1981) ano Glennie (1984, 1986). Harsh desert erosion, including the high run-off rate of rain attracted by the marked initial relief, doubtless led to rapid downwasting during the first few million years of the early Permian and to the accumulation of considerable thicknesses of screes and water-laid valley-fill; little sediment of this initial immature phase now survives (Wills 1948; Smith 1972), having been swept away as continuing erosion cut below the level of the early valley fill. For this reason, the sedimentary record across the system boundary probably is not continuous except perhaps in a few places in parts of the English Midlands and southwest England where the main phase of downwasting may have been Stephanian; an hiatus of at least one stage thus generally separates Carboniferous and Permian strata, even in the Mauchline Basin of Scotland where the rocks appear to be conformable. It was presumably during this depositional hiatus that the early Permian land surface was widely reddened to depths generally of a few metres but locally to 300 m. Early Permian strata throughout the British Isles have yielded only a few traces of contemporary life, probably mainly because of the low potential for the preservation of fossils in desert environments. As Paton (1974) pointed out, however, the presence of two species of carnivorous pelycosaurs in early Permian strata in the English Midlands must indicate a substantial food chain, including at least some herbivores and desert plants. The shortage of fossils precludes reliable correlation between rocks in the various basins, and inferences of approximate contemporaneity are doubtfully based on a few reptilian footprints, on the similarities between broad sedimentary sequences and, in a few places, on the first appearance of certain marker pebbles. The data are too fragmentary and poorly dated to provide a clear palaeogeographic image of the whole of the British Isles during this first part of the early Permian and a map depicting this phase has not been attempted.
L a t e early P e r m i a n The palaeogeography of much of the British Isles during late early Permian time is reasonably well attested by desert deposits, although there is virtually no evidence relating to large parts of Ireland, Scotland, Wales and southeast England; these areas presumably remained as barren desert highlands, supplying detritus to valleys and thence to surrounding and intervening lowlands and sedimentary basins. Where sedimentary evidence has survived, the depositional patterns differed considerably between western and eastern parts of the British Isles. In the west, sedimentation appears to have continued during the late early Permian in the pattern already established during the early early Permian and, in places, the late Carboniferous. Angular rock debris accumulated in countless screes and valleys, whence it was swept by debris flows and flash floods into sedimentary troughs and basins so as to form vast complexes of coalescing alluvial fans (bajadas); in the larger troughs and basins there is evidence that some sediments were protected from further erosion by synsedimentary subsidence and that fine-grained sediments (and locally also evaporites) accumulated at times in basin centres such as that of the East Irish Sea Basin. Vivid word pictures of desert sedimentation in western Britain have been drawn in terms of modern desert sedimentary processes by, amongst others, Brookfield (1980) for southwest Scotland and Laming (1965) for southwest England. Very little information is available on the nature and thickness of late early Permian strata in basins off the north and west coasts of Scotland and Ireland but for the construction of our maps of these areas we have assumed that these strata are generally similar in character to those in the basins onshore. Although many western parts of the British Isles remained as upstanding sediment source areas during late early Permian, sedimentological evidence suggests a general decline in relief and a merging of basins as interfluves were reduced and sediments accumulated. With declining relief came a reduction in the rate and carrying capacity of run-off, and wind transport of detritus became dominant in almost all the western basins; with little vegetation to hinder its passage, wind-blown sand collected in basins from western Scotland to the southermost part of England. Prevailing easterly and northeasterly winds have been inferred from cross-lamination trends in most western basins, (e.g. Shotton ! 956; Steele 1981), but Sneh (1988) alternatively inferred prevailing northerly winds in several of these. In the east of the British Isles and in much of the North Sea area, information from surface exposures and many boreholes suggests that 87
88
PERMIAN
the landscape had evolved by late early Permian time into an extensive and gently rolling rock peneplain upon which, except perhaps in eastern Scotland, few major hills survived. Sediments of the early phases of rapid downwasting had been removed or recycled, leaving only a widespread thin desert pavement of residual rock debris (some far-travelled) with a sandy matrix that included many trapped wind-blown grains. This thin desert breccia remains the only early Permian deposit in parts of east and northeast England and on much of the mid North Sea High, and it persists eastwards and southwards beneath later early Permian sediments in parts of the Northern and Southern North Sea basins; in these basins, however, continuing broad gentle subsidence led to the creation of major inland drainage basins (Glennie 1972), some with floors inferred to have been well before contemporary sea level (Smith 1970; Glennie & Buller 1983) in which desert sediments accumulated to thicknesses locally exceeding 300 m. These sediments, the equivalent of the Rotliegend and Weissliegend deposits of Germany, were formed in a wide range of desert environments including extensive hypersaline lakes, inland sabkhas and aeolian sand seas (Glennie 1972, 1984). As in western areas, fluvial sedimentation was dominant in the earlier phases and near the basin margins and was widely superseded by aeolian sedimentation in the latter part of the late early Permian. There is, however, some evidence to suggest an increase in rainfall and an expansion of desert lakes shortly before the Zechstein transgression. Despite the difficulties of correlation in the almost barren desert sediments, the data for many areas permit a reasonable attempt at depicting the late early Permian palaeogeography of the British Isles. See also Wills (1973). DBS, MEB, KWG LATE
number of sub-cycles; the rocks of the English part of the Zechstein Sea basin comprise four main cycles (EZI-4), one minor cycle (EZ5) and several sub-cycles. Overlying red continental mudstones and siltstones (some perhaps Triassic) in the deepest North Sea depocentres also include a number of thin but extensive lenses of ?lacustrine evaporites that might be designated EZ6-8. For the purposes of map construction we have accepted that strata of Cycles 1 to 3 of the Bakevellia Sea Basin probably correlate with those of Cycles 1 and 2 of the English Zechstein Basin (Smith et al. 1974; Arthurton et al. 1978) and that cycles 3 and 4 of the English Zechstein are represented in the Bakevellia Sea Basin mainly by red continental strata and basin-centre playa or limnic evaporites. There is little published information on late Permian strata in any of the basins off the west coasts of Scotland and Ireland, but a Zechstein microflora and marine microfossils have recently been reported from 20 to 30 m of grey and red elastics, dolomite, gypsum and anhydrite in boreholes in the Erris and Donegal basins off northwest Ireland (Tate & Dobson 1989).
PERMIAN
Continental sedimentation continued during the 5 million or so years of the late Permian in parts of most land areas of the British Isles, though probably more slowly than in the early Permian because of reduced relief and perhaps lower rainfall. Elevated sediment-source areas remained extensive and continued to shed coarse rock debris, but most late Permian continental sediments accumulated in the existing low-lying areas and were mainly alluvial sands, silts and clays (now red); these sediments widely superseded the early Permian aeolian deposits but aeolian transport and sedimentation doubtless continued in places. In contrast, the inferred rapid flooding from the north of the sub-sea level inland drainage basins in the northeast and northwest of Britain (Smith 1970a,b, 1979; Steele 1981; Giennie & Buller 1983) led to the formation there of, respectively, the Zechstein Sea and the smaller and shallower Bakevellia Sea (Fig. 1). Sedimentological evidence suggests that the Zechstein Sea filled an existing depression perhaps 200 to 250 m deep in which synsedimentary subsidence led to the accumulation of more than 2 km of late Permian cyclic carbonate, evaporite and siliciclastic rocks (Fig. 2); equivalent strata in the Bakevellia Sea Basin are more than 300 m thick (Jackson et al. 1987).
Fig. 2. Generalized isopachyte map of Zechstein strata in the North Sea and adjoining areas. Modified after Taylor (1986--modification after Taylor 1981). Reproduced by permission of Blackwell Scientific Publications Ltd. The mutual relationships of the various stratigraphical units of the late Permian sequence in northeast England are shown in Fig. 3, using the new nomenclature proposed by Smith et al. (1986) for the Yorkshire (southern) province of the English Zechstein outcrop; the names of equivalent strata in the Durham (northern) Province and in the Southern North Sea and adjoining basins are shown in brackets in Fig. 3 and also in Table 1. DBS, JCMT
P2a: Lower part of English Zechstein Cycle 1 and equivalents
Fig. !. Late Permian seas of northwest Europe and adjoining areas. Modified after Taylor (1986). Reproduced by permission of Blackwell Scientific Publications Ltd. The rocks of the Bakevellia Sea Basin have been divided into three main cycles by Smith et al. (1974) following Arthurton & Hemingway (1972) or four cycles (BS 1-4) by Jackson et al. (1987), and probably also include a
Map P2a depicts the palffeogeography of the northern part of the British Isles as we envisage it towards the end of the carbonate phase of the second subcycle of the English Zechstein sequence. At this time, rather more than half of the area was land and the remainder was covered by the newly formed epicontinental seas. Land areas remained much as in the late early Permian, with coarse detritus forming screes and alluvial fans in the many surviving hilly areas in the west and north and being broken down and transported to isolated or interconnected valleys, inland drainage basins and cuvettes where sands, silts, clays and perhaps evaporites accumulated on slowly widening alluvial plains; limited evidence suggests that sporadic fluvial sediment transport processes greatly predominated in most areas, perhaps because of a reduction in wind transport resulting from the establishment of plant cover, but that wind continued to move such silt and fine sand. The geography of southern parts of the British Isles (not shown on Map P2a) probably persisted much as depicted in Map PI except for the widespread formation of fluvial sedimentary plains and perhaps lakes where sand seas had previously stood, such plains probably were the depositional environment of the sediments of the Aylesbeare Group in Devon, for
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
90
PERMIAN
Table !. Correlation of late Permian rocks in northeast England with those of North Germany and Holland. After Smith (1989), reproduced with permission of the Yorkshire Geological Society.
GROUPS
~
(J
ESKDALE EZ5, GROUP /
YORKSHIRE PROVINCE (OUTCROPAREA)
DURHAM P R O V I N C E (County Durham, east Tyne and Wear, County Cleveland)
Y O R K S H I R PROVINCE E (East and North Yorks and Humberside)
ROXBY FORMATION
ROXBY FORMATION
STAINTONJEz4 SHERBURN ANHYDRITE DALE UPGANG FORMATION GROUP /" ROTTENMARL
SHERBURN ANHYDRITE UPGANG FORMATION ROTTEN MARL
BILLINGHAM ANHYDRITE TEESSIDE GROUP EZS: BROTHERTON FM
TT-I ?[-]-FT1 ?r T F T ~ ? , - AISLABY GROUP
EZ2i EDLINGTON FORMATION
DON
3::E=== Eb Sprotbrough Member~ CADEBY
G ROU p
~'FORMATION ~-a Wetherby Member J
BOULBY HALITE BILLINGHAM ANHYDRITE SEAHAM FORMATION
..,_,__.
NORTH GERMANY AND HOLLAND
"6 (~
LITTLEBECK ANHYDRITE SLEIGHTS SILTSTONE
OHRE ANHYDRIT UNTERER OHRE TON
Z5
SNEATON HALITE SHERBURN ANHYDRITE UPGANG FORMATION CARNALLITIC MARL
ALLER SALZE PEGMATITAN HYDRIT Thin unnamed carbonate ROTER SALZTON
Z4
BOULBY HALITE BILLINGHAM ANHYDRITE BROTHERTON FORMATION GRAUER SALZTON
LEINE SALZE HAUPTANHYDRIT PLA'I-I'ENDO LOMIT GRAUER SALZTON
Z3
1,
SEAHAM RESIDUE
FORDON EVAPORITES
ROKER DOLOMITE AND CONCRETIONARY LIMESTONE
KIRKHAM ABBEY FORMATION
HAUPTDOLOMIT A N D EQUIVALENTS
HARTLEPOOL ANHYDRITE
HAYTON ANHYDRITE
WERRAANHYDRIT
CADEBY FORMATION
ZECHSTEINKALK
MARL SLATE
KUPFERSCHIEFER
STASSFU RT SALZE BASALANHYDRIT
Z2
FORD FORMATION RAISBY FORMATION
MARL SLATE
MARL SLATE
BASAL PERMIAN (YELLOW) SANDS AND BRECCIAS
YELLOW (BASAL PERMIAN) SANDS AND BRECCIAS
example, and of similar formations in late Permian basins elsewhere, though even approximate contemporaneity cannot be proved. The relief of large parts of southern and southeastern Britain appears to have become relatively subdued at this time, with maturing peneplains cloaked only locally with debris (now piedmont breccias) and supplying little sediment to adjoining plains and seas. Red hues were probably widespread everywhere in the many areas of bare rock, but it is unclear whether the patchy sedimentary mantle was initially red or whether the resulting rocks acquired their present redness during and after burial. Our placing of the shorelines of the epicontinental seas ranges from firm to speculative according to the evidence available and our estimate of contemporary relief; much of the evidence has been eroded from large areas. In the west, the western outline of the Bakevellia Sea is tentatively based on assumptions of palaeorelief and sea levels and our reconstruction of lithofacies is similarly speculative; eastern outlines and facies of the Bakevellia Sea are more firmly established by outcrop and borehole data but much uncertainty remains. Lithofacies of the Bakevellia Sea Basin (Map P2a) have been reconstructed on the assumption that the Saltom (first) and Sandwith (second) cycles there roughly equate with the first and second Zechstein subcycles of eastern England. The positions of southern and southwestern shorelines of the English Zechstein Sea have been estimated from outcrop evidence and much borehole information and in places are reasonably firm, but our positioning of the western shoreline off Scotland is uncertain. There is no evidence of a direct cross-Pennine link between the Bakevellia and Zechstein Seas at this time, both apparently having been fed from the north (Smith 1970a). The carbonate rocks of both sub-c3)cles of the English Zechstein Cycle 1 in northeast England comprise several lithofacies bel~s (Smith 1974, 1980, 1989; Taylor & Colter 1975; Harwood 1981), but the small scale of Map P2a permits only some of these to be depicted. The original limestones are mainly dolomitized. In the first sub-cycle (not shown o,'n Map P2a), a broad shelf prism of mainly shallow-water ooidal grainstgnes, grainstones and patch reefs was prograded over a suite of basin-ma(gin slope wackestones and mudstones and both lithofacies grade downslope (i.e. eastwards) into starved or semi-starved basin-floor calcite mudstofies (some laminated). A minor sea level fall and recovery separated the first from the second subcycle (Smith 1968) and facies in the latter were much influenced by the inherited sea-floor topography; thus, a linear reef or equivalent steep slope separated a broad shelf on which ooid grainstones and packestones predominated, from thin basin-floor mudstones. Ooid sandwaves (Smith 1974a; Kaldi 1980, 1986) comprised a major N-S belt in most southern outcrop areas. A number of offshore platforms and islands persisted throughout the cycle and led to patchy shoals and narrow fringing shelves. The youngest part of the cycle, not shown on Map 2a, is a distinctive thin bed of oncoids and columnar stromatolites (Smith 1970b, 1986; Taylor & Colter 1975) that covers higher parts of the basin floor and is also known in Germany and Poland.
P2b: Upper part of English Zechstein Cycle 1 and equivalents This map outlines the geography of the north of the British Isles during the upper part of the sulphate phase of English Zechstein Cycle 1; it assumes.
BASAL PERMIAN SANDS AND B R E C C I A S
Zl
ROTLIEGENDES
following Smith et al. (1974), that the Lower Gypsum of Kingscourt (Eire), the Sandwith and Haverigg Haws anhydrites of west and south Cumbria and the B-Bed sulphate rocks of the Vale of Eden are all approximate correlatives of the EZI anhydrite. Evidence suggests that sea level fell by at least a few metres following Cycle 1 carbonate deposition, and that this fall probably resulted in widespread emergence of the carbonate shelves and a corresponding increase in the width of coastal plains. Apart from this increase and a probable slight further lowering of the overall relief, we believe that the landscape and climate of all land areas of the British Isles (including those parts not shown in Map P2b) remained much as they were during the carbonate phase of this cycle; the supply of terrigenous sediment to the marine basins remained low, being mainly of (?wind-blown) silt and fine sand. Mindful that the original extent of Permian evaporites has commonly been greatly reduced by dissolution, our placing of the evaporite margins and lithofacies owes much to information on modern supposed analogues. The uncertainties of this unavoidable approach are compounded in western parts of the Bakevellia Sea Basin where the records of the lithology of anhydrite and gypsum are too imprecise to allow interpretation of their depositional environment. The detailed records of the anhydrite in boreholes in west Cumbria (Arthurton & Hemingway 1972), however, permit the interpretation of both subaqueous and sabkha-type depositional environments near the steeply shelving shoreline and provide a reasonably firm basis for our lines there. Similarly detailed descriptions of parts of the 'B-Bed' sulphate of the Vale of Eden also indicate both subaqueous and sabkha-type deposition there, possibly with a marine connection northwards to the Carlisle Basin (Arthurton 1971; Burgess & Holiday 1974; Arthurton et al. 1978). Our estimate of the depositional margin of the Cycle I anhydrite in northeast England allows for some basinward retreat through dissolution, but we do not know whether the anhydrite formerly extended onto the Cycle 1 carbonate shetf (as appears likely in places) or was restricted to the area basinward of the Cycle I carbonate shelf-break; whichever is correct, the main area of Zechstein sulphate deposition was probably surrounded by brine-soaked coastal plains where sulphate accumulated when and where conditions were suitable. Within the basin shelf-break, thick massive mainly nodular anhydrite forms a broad belt (Fig. 3 and Taylor & Colter 1975, fig. 3C) and passes basinward into a few metres of interbedded laminated carbonate and anhydrite in which a number of beds of nodular anhydrite also occur (Taylor & Colter 1975; T,aylor 1980, 1984). There is general agreement that the basin-floor laminites were subaqueous, with the nodular layers possibly indicating phases of deep drawdown or complete desiccation (Taylor 1980). By contrast, the nodular fabric of the massive marginal anhydrite is capable of several environmental interpretations; that shown in map P2a takes the view that the sulphate was formed mainly by displacive and replacive growth in sabkha sediments whereas an alternative view would be that the nodular fabric is a diagenetic modification that has obscured a primary (possibly subaqueous) fabric. Massive halite forms a lens within the Cycle 1 anhydrite off the Norfolk coast and may be of salina origin (Taylor 1986). DBS, JCMT, RSA
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
92
PERMIAN
WEST
EAST
100 metres
L I T T ~ . ~ . ~~176
O kilometres
~,O/'.6 '"(
SLEG I ---~ HTSS I L T S T O ~ -- ~~ sNEATONHALITE ~ .
EZ5 /1EZ4
I
~ C A R N A L L I T I C MARL ~ ] -'-"-"BOULBYHALITE BoulbyPotash / ~ ~ BILLINGHAMA~HYDRI.___.__~EZ3 _~L BROTHERTON(SEAHAM)FORMATION -GrauerSalzton / EDLINGTON 2 ---?~-~Sub-cycle3 FORMATION ~ 7 O0 /c/~, -.....~'\-. -1 Biostrome " ~O,, - ~C4'.qa. ~'-~"~FORDONEVAPORITES k. ' ~ Erosionsurface ~ /O4~~/Fw~?~z. x~L.ISEAHAMRES,DUE) SPROTBROUGHMEMBER ,~" _ ~oyo~o: %,_ \ . Sub-cycle 2 (FORDFORMATION)surface .~~o. k ~ ~ "'4r162 " N ' ~ O ~ TM. 'EZ2 "---'-',--~,tEr~176 ; o IHAYTON(HARTLEPOOL) ~ "O4~'ff4tdrOlk " ~ P o l e - - . " WETHERBYMEMBER ~ ~ ( R A I FO SRMABTION)Y
~
MARLSLATE
~ PointBed
.EZ1
Fig. 3. Spatial relationship of late Permian lithostratigraphical units of northeast England: nomenclature after Smith et al. (1986). After Smith (1989), reproduced with permission of the Yorkshire Geological Society.
P3a: Lower part of English Zechstein Cycle 2 and equivalents Map P3a depicts the palaeogeography of the northern part o f the British Isles towards the end of the carbonate phase of Cycle 2 of the English Zechstein sequence. The marine transgression that initiated this cycle was less extensive than that of Cycle 1, and the seas, particularly in the west, were probably smaller. For construction purposes we have followed Smith et al. (1974) in correlating the EZ2 carbonates in the east with the thin Avoniel Limestone in Northern Ireland and with the thin Fleswick and Roosecote Dolomites in west and south Cumbria; possibly-equivalent carbonate rocks have been reported in the Irish Sea only in Well 110/41, about 10 km off Blackpool (Jackson et al. 1987). Land areas of Britain have been depicted on Map 3a as having evolved slightly from those shown in Maps P2a and P2b, on the assumption that desert and semi-desert erosion Continued to wear down the uplands and the resulting debris was transported to ever-broadening alluvial plains and playas. Coarse detritus continued to be shed into intermontane valleys and troughs such as the Vale of Eden and several of those in and off western Scotland (Brookfield 1978) and elsewhere in western Britain, and breccias and other continental deposits are interbedded with marine strata in a few places such as west Cumbria where the late Permian coastlines were steep; most late Permian continental sediments, however, cannot be dated precisely and we have assumed that the patterns and rates of sedimentation remained broadly constant but evolved gradually during the life of the Bakevellia and Zechstein seas. East Anglia and much of southeast England appear to have been low-relief sediment-source areas almost throughout. Little information is available on the lithology of rocks thought to be of this age in the Bakevellia Sea Basin and our assessment of depositional environments and facies is correspondingly vague here. Borehole data from parts of the East Irish Sea (Jackson et al. 1987), however, appear to suggest that rocks o f this age may be generally thin and may comprise interbedded carbonates and siliciclastics. In contrast, carbonate and equivalent Cycle 2 strata of the English Zechstein form a reasonably well documented series of belts in northeast England (Smith 1974, 1989; Taylor & Colter 1975; Taylor 1984). These comprise carbonate belts including shelf (shoal), platform, slope and basinfloor lithofacies (see Fig. 3 for relationships and nomenclature), and a marginal belt in which occurs a complex mosaic of interdigitating coastal plain and lagoonal siliciclastic, evaporite and carbonate lithofacies. The marginal belt approximately coincides with the position of the Cycle ! carbonate shelf, and the shoal facies prograded basinwards over the Cycle I anhydrite and Cycle 2 slope carbonates. Shoals were also widespread across the mid-North Sea High which was, however, breached by a number of channels or passages (Jenyon et al. 1984; Smith & Taylor 1989). Sedimentation of the carbonate rocks of this cycle was strongly influenced by an
oscillating but basin-wide pycnocline, below which only relatively thin carbonaceous carbonate laminites were deposited widely across an anoxic and almost featureless basin floor up to perhaps 300 m deep. DBS, JCMT, RSA
P3b: Upper part of English Zechstein Cycle 2 This map represents the late Permian geography of the northern part of the British Isles late during the formation of Cycle 2 Zechstein evaporites. We have followed Smith et al. (1974) in correlating the thick salt of Larne No 2 Borehole (Penn 1981) and the East Irish Sea (Colter & Barr 1975; Jackson et al. 1987) with the thick Cycle 2 Zechstein salts and we suspect that salt of this age may also have been precipitated in a number of the subsiding depocentres in the Celtic Sea and perhaps also in southern Britain; brine concentration seems not to have advanced beyond the sulphate phase in most parts of the Bakevellia Sea Basin (unless salt has later been dissolved) and it is arguable that up to 15 m of sulphate rocks in several circum-Irish Sea localities (Kingscourt, Tyrone, Belfast, Cumbria, etc) should also be correlated with the late Cycle 2 halite. We assume that land areas of the British Isles expanded slightly during this evaporitic phase, in response to episodic shrinkage of the northern seas, and our portrayal of these areas takes account of continued erosion of uplands and further widening of alluvial plains. A lack of detailed information of the fabrics of the evaporites in many western parts of the Bakevellia Sea Basin precludes reliable environmental reconstruction and here we have postulated a range of coastal plain and shallow subaqueous depositional environments; the Larne halite occupies the depocentre of a major basin, and here we envisage extensive salt flats, surrounding a broad salina or playa that fluctuated widely in depth and extent and which must, from time to time, have been flooded with sea water. A similar depositional environment seems likely for the thick halite of the East Irish Sea sub-basin. Detailed observation of the Fleswick Anhydrite of west Cumbria allows reasonable inferences of marine-marginal sabkha-type and shallow-water sedimentation there (Arthurton & Hemingway 1972), and similarly careful recording of the supposedly equivalent C-Bed sulphate of the Vale of Eden (Arthurton 1971; Arthurton et al. 1978) allows an inference of mainly shallow water (?marine-marginal or lagoonal) deposition. In the east, the later stages of Cycle EZ2 saw the deposition of enormous volumes of evaporites, mainly halite, that reached 200 m in thickness just inside the basin shelf-break and widely exceeded 300 m in basin depocentres (Taylor & Colter 1975). These rocks, the Fordon or Stassfurt Evaporites, comprise several sub-cycles (Stewart 1949, 1953, 1963; Colt er & Reed 1980) and must have had access to oceanic brines. Anhydrite rocks reported by Stewart include cross-bedded ooidal varieties, delicate laminites and others rich in pseudomorphs after bottom-grown gypsum, and phases of sub-
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
94
PERMIAN
aqueous marine deposition must be inferred; these varieties moreover, are intimately associated with halite containing fabrics compatible with (but not necessarily diagnostic of) salt-fiat deposition. The petrographic evidence on the depositional environment partly conflicts with the geometry of the basin and of the evaporite body and we allow the possibilities of both deep and shallow-water subaqueous deposition as well as the progradational growth of marginal shelves as argued by Colter & Reed (1980); periodic desiccation and resulting deep drawdown may be indicated. In northeast England the Cycle 2 evaporites extend as a sharply thinned sheet (see Fig. 3 for geometry) for many kilometres across the broad shelf of the Cycle 2 carbonates and here bear unmistakable evidence of shallow water formation; their futher landwards extension cannot yet be estimated, but they may have merged with evaporites already being deposited in the coastal plain-lagoonal complexes landward of the Cycle 2 carbonate shelfbarrier (Map P3a). DBS, JCMT, RSA
P4a: Lower part of English Zechstein Cycle 3 and equivalents This map portrays the geography and lithofacies of late Permian rocks in the northern part of the British Isles towards the end of the carbonate phase of Cycle 3. It accepts approximate equivalence of the Belah Dolomite of the Vale of Eden with the Cycle 3 carbonate of eastern England (Burgess 1965) and possible equivalence with carbonate rocks of about this age in the Bakevellia Sea Basin (see Jackson et al. 1987). The initial marine transgression of Cycle 3 locally extended beyond that of Cycle 2 and, in places, also beyond that of Cycle 1; this is probably partly because of the gradual erosion of land areas and partly because of slight northwards tilting and continuing subsidence. For our purposes we have assumed that landscapes in all land areas continued to mature and relief to lessen, with sedimentation persisting patchily in the same areas as previously; correlation of the resulting continental rocks in, for example, southwest England with those of the northern marine sequences cannot yet be proved, but we have assumed a general equivalence. The clay mineralogy of the Littleham mudstones of the higher part of the later Permian sequence in south Devon is consistent with deposition on a desert-marginal flood plain (Henson 1973), the inferred primary source of the clays being ferruginous highland soils produced by intensive weathering in a warm climate with alternating wet and dry seasons (Cosgrove 1972). Supposedly equivalent continental sediments in the smaller basins such as some of those in southwest Scotland were mainly water-laid sands and gravels (Brookfield 1978, 1980), consistent with their local provenance and greater surrounding relief. Because of doubts regarding correlation and lithofacies, it is not yet clear whether the Bakevellia Sea re-flooded central and eastern parts of the Irish Sea basins during the carbonate phase of English Zechstein Cycle 3. Most boreholes both on land and offshore report red siltstones at about this level and we have accordingly shown much of the basin as a vast sedimentary plain at this time. Jackson et al (1987) however, report thin dolomites in two boreholes in the East Irish Sea Basin and infer a large basin-centre playa; other playas may have existed. Cycle 3 carbonate rocks in northeast England and the North Sea comprise a broad marginal belt that thickens from a feather-edge to about 75 m before thinning sharply to a basin-floor sheet (Taylor & Colter 1975; Taylor 1986). The mainly dolomite lithology is more uniform than in the carbonate rocks of Cycles I and 2, in response to less varied sea-floor topography, and we infer a generally shallow epeiric sea of slightly enhanced salinity. In the south the carbonate rocks die out in a sequence of red coastal-plain siltstones and sandstones, mudstones, and in the west the Cycle 3 sea extended across Stainmore and a short distance up the Vale of Eden (Burgess 1965; Burgess & Holliday 1974). DBS, JCMT, RSA
connection, is envisaged as the depositional environment of the D-Bed sulphate of the southern part of the Vale of Eden (Arthurton 1971; Burgess & Holliday 1974). A full sequence of evaporites preceded the formation of the Cycle 3 potash salts in northeast England, but their present distribution owes much to dissolution and the contemporary shorelines are difficult to reconstruct; in Map P4b we have deduced these depositional limits from known westwards thinning rates in areas thought not to have been affected by dissolution. The rocks of the sulphate phase were probably the most extensive and were formed in a range of marginal sabkha-type and lagoonal environments that interfinger basinwards with increasing proportions of subaqueously-formed gypsum/anhydrite. Subsequent halite rocks appear mainly to have been formed on enormous salt flats surrounding an inferred marine-connected hypersaline lagoon or shallow sea (Smith 1974, 1980), and the potash salts are judged initially to have been formed in marginal lagoons and hollows in the salt fiat and, in the closing stages of the cycle, in residual basin-centre evaporite pans (Smith 1973; Smith & Crosby 1979). Carnallite was probably the main primary potash mineral (Stewart 1956). DBS, JCMT, RSA
P4c: Upper middle part of English Zechstein Cycle 4 and equivalents In this map we reconstruct the geography of central parts of the British Isles during the deposition of Cycle 4 potash salts (EZ4K) in northeast England; this departure from our previous practice is to take account of the inferred greater depositional variety at this time than towards the beginning and end of the cycle. As in Cycle 3, shortage of information and the uncertainty of correlations preclude detailed reconstruction of environments in northern and southern parts of the British Isles but we assume a further general topographic evolution along the lines depicted for these areas during English Zechstein cycles 1 to 3. From the apparent absence of carbonate and evaporite rocks of this age in. the Bakevellia Sea Basin, it is not yet possible to establish firm correlations here with Zechstein sequences in northeast England. In all areas for which records are available, however (Kingscourt, the Tyrone and Belfast areas of Northern Ireland, the East Irish S e a a n d west and south Cumbria), the whole of the relevant part of the sequence is of red sandstone (subordinate), siltstone and mudstone, and we have portrayed environments here as mainly fluvial distal sedimentary plains; doubtless ephemeral playas occurred in some depocentres and other inland drainage areas and these may, from time to time, have been the locus of temporary evaporite accumulation. Similar lithofacies characterize the Vale of Eden, where, additionally, an aeolian origin has been inferred for parts of the sequence (Burgess & Holliday 1974). As in Cycle 3, a full evaporite sequence is present in northeast England, where two sub-cycles may be recognized (Smith 1971); a fully marine environment has not been proved during any part of the cycle, however, and the initial magnesitic carbonate unit is everywhere less than 2 m thick and may be limnic. The succeeding sulphate unit is one of the most distinctive and widespread units of the English Zechstein sequence, and appears to have been formed mainly or entirely subaqueously (Stewart 1951b). Dissolution has reduced the extent of the Cycle 4 anhydrite by less than that of its Cycle 3 counterpart, but has greatly reduced the marginal extent of the Cycle 4 halite; here our map makes some allowance for dissolution. The halite itself appears to have been formed mainly on extensive salt fiats and a similar depositional environment has also been envisaged for the Cycle 4 potash salts (Smith 1973). The youngest major part of the Cycle 4 salt sequence is of the haselgebirge type, inferred to have been formed by crystallization within the brine-soaked sediments of a wide prograding coastal plain (Smith 1971); basin-centre equivalents probably were deposited in a range of salt-fiat and saline environments and include thick potash salts (Smith & Crosby 1979). DBS, JCMT, RSA
P4b: Upper part of English Zeehstein Cycle 3 and equivalents P4d: English Zechstein Cycle 5 and equivalents This map depicts the geography of part of the British Isles whilst potash salts were being formed in northeast England and the North Sea. The D-Bed sulphate of the Vale of Eden may be equivalent or slightly older, and interbedded anhydrite, halite and clastic rocks in Well 110/8-2 in the East Irish Sea depocentre (Jackson et al. 1987) may be generally contemporary. Information in the extreme north and south of the British Isles is too sparse and correlation too uncertain to allow the construction of useful palaeogeographic maps of those areas but we presume that landscapes there continued to advance towards maturity as erosion and sedimentary infilling continued. Despite the great duration (perhaps 40 to 50 million years) of this continental phase, it is clear that substantial relief and the shedding and deposition of coarse detritus persisted to the end of the Permian in many western areas (including those shown on Map P4b); much of southeast England, however, was approaching a mature phase of peneplanation, and most continental sediments throughout Britain were fine grained. We have no evidence of a marine influence in the Bakevellia Sea Basin at this time and we envisage here the persistence of the vast sedimentary plain inferred during the Cycle 3 carbonate phase (Map P4a); the presence of salt and anhydrite in Well 110/8-2, if correctly correlated with the Cycle 3 evaporites of eastern England, implies the presence of a hypersaline lake or playa in the depocentre of the East Irish Sea sub-basin, and there may have been others. A similar lake or playa, though perhaps with a lingering marine
This brief incomplete evaporite cycle, the last of the English Zechstein sequence in northeast England and adjoining areas, appears to be represented in all other late Permian sedimentary basins by fluvial and lacustrine siltstones and mudstones. We assume that landscapes and sedimentary patterns here differed little from those established earlier. Cycle 5 in northeast England is represented solely by the Littlebeck Anhydrite, a thin layered rock that appears to be of subaqueous origin. The great width of the outcrop (Map P4d) is surprising but may be a result of contraction of the hypersaline lake or sea in which it was formed. Succeeding salts in basin-centre areas include a few metres of halite and anhydrite (Taylor & Colter 1975; Taylor 1984). Little is known of the lithology of these rocks and we cannot, therefore, assess their depositional environments. DBS, JCMT
Acknowledgements.Assistance in this compilation was given by D. Evans, R. McQuillan and D. K. Smyth (offshore Scotland), J. P. B. Loveil (Scotland), E. H. Francis (Midland Valley of Scotland), A. E. Griffith (Northern Ireland), M. K. Lee (Solway Firth Basin), D. 1. Jackson (East Irish Sea Basin), R. A. Old (English Midlands), G. Kirby, B. W. Sellwood and. A. Whittaker (southern England) and P. R. R. Gardiner (Eire). Mrs D. G. Evans drafted two of the text figures.
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
96
PERMIAN
References ARTHURTON, R. S. 1971. The Permian evaporites of the Langwathby Borehole, Cumberland. Rep. No. 71/7, Institute of Geological Sciences. - - , BURGESS,I. C. & HOLLIDAY, D. W. 1978. Permian and Triassic. In: MOSELEY, F. (ed.) The Geology of the Lake District. Yorkshire Geological Society, 189206. -& HEMINGWAY,J. E. 1972. The St. Bees Evaporites--a carbonate--evaporite formation of Upper Permian age in west Cumberland, England. Proceedings of the Yorkshire Geological Society, 3, 565-592. BROOKFIELD, M. E. 1978. Revision of the stratigraphy of Permian and supposed Permian rocks of southern Scotland. Geologischen Rundschau, 67, 110-149. 1980. Permian intermontane basin sedimentation in southern Scotland. Sedimentary Geology, 27, 167-194. BURGESS, I. C. 1965. The Permo-Triassic rocks around Kirkby Stephen, Westmorland. Proceedings of the Yorkshire Geological Society, 35, 91-101. -& HOLLIDAY,D. W. 1974. The Permo-Triassic rocks of the Hilton Borehole, Westmorland. Bulletin of the Geological Survey of Great Britain, 46, 1-34. COLTER, V. S. & BARR, K. W. 1975. Recent developments in the geology of the Irish Sea and Cheshire basins. In: WOODLAND, A. W. (ed.) Petroleum and the Continental Shelf of North-West Europe. Applied Science, Barking, 61-73. & REED, G. E. 1980. Zechstein Fordon Evaporites of the Atwick No. 1 borehole, surrounding areas of N.E. England and the adjacent southern North Sea. Contributions to Sedimentology, 9, 115-129. COSGROVE, M. E. 1972. The geochemistry and mineralogy of Permian red beds of southwest England. Chemical Geology, 11, 31--47. GLENNIE, K. W. 1972. Permian Rotliegendes of north-west Europe interpreted in light of modern desert sedimentation. Bulletin of the American Association of Petroleum Geologists, 56, 1048-1071. 1983a. Early Permian (Rotliegendes) palaeowinds of the North Sea. Sedimentary Geology, 34, 245-265. 1983b. Lower Permian Rotliegend desert sedimentation in the North Sea area. In: BROOKFIELD, M. E. & AHLBRANDT, T. S. (eds) Eolian Sediments and Processes. Developments in Sedimentology, Elsevier, Amsterdam, 521-541. 1984. Early Permian-Rotliegend. In: GLENNIE, K. W. (ed.) Introduction to the Petroleum Geology of the North Sea. Blackwell, Oxford, 41~50. & BOEGNER,P. 1981. Sole Pit inversion tectonics. In: ILLING, L. V. & HOBSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 110-120. & BULLER,A. T. 1983. The Permian Weissliegendes of N.W. Europe: the partial deformation of aeolian dune sands caused by the Zechstein transgression. Sedimentary Geology, 35, 43-81. HARWOOD, G. M. 1981. Controls of Mineralization in the Cadeby Formation (Lower Magnesian Limestone). PhD thesis, Open University. HENSON, M. R. 1973. Clay minerals from the Lower New Red Sandstone of South Devon. Proceedings of the Geologists" Association, 84, 429-445. JACKSON, D. I., MULHOLLAND, P., JONES, S. M. & WARRINGTON,G. 1987. The geological framework of the East Irish Sea Basin. In: BROOKS,J. & GLENNIE, K. W. (eds) The Petroleum Geology of North West Europe. Graham & Trotman, London, 193-203. KALDI, J. G. 1980. Aspects of the Sedimentology and Diagenesis of the Lower Magnesian Limestone (Permian) of Yorkshire, England. PhD thesis, University of Cambridge. 1986. Sedimentology of an oolite shoal complex in the Cadeby (Magnesian Limestone) Formation (Upper Permian) of eastern England. In: HARWOOD, G. M. & SMITH, D. B. (eds) The English Zechstein and Related Topics. Geological Society, London, Special Publication, 22, 63-74. KENT, P. E. 1980. Subsidence and uplift in East Yorkshire and Lincolnshire: a double inversion. Proceedings of the Yorkshire Geological Society, 42, 505-524. LAMING, D. J. C. 1966. Imbrications, palaeocurrents and other sedimentary features in the Lower New Red Sandstone, Devonshire, England. Journal of Sedimentary Geology, 36, 940-959. PATON, R. L. 1974. Lower Permian Pelycosaurs from the English Midlands. Palaeontology, 17, 541-552. PENN, I. E. 1981. Larne No. 2 Geological Well Completion Report. Report of the Deep Geology Unit, Institute of Geological Sciences, No. 81/6. RHYS, G. H. (compiler) 1974. A Proposed Lithostratigraphic Nomenclature for the -
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Southern North Sea and an Outline Structural Nomenclature for the whole of the (UK) North Sea. Report/Institute of Geological Sciences, 74/8. RUSSELL, M. J. 1976. A possible Lower Permian age for the onset of ocean floor spreading in the northern North Atlantic. Scottish Journal of Geology, 12, 315323. & SMYTHE, D. K. 1978. Evidence for an early Permian oceanic rift in the northern North Atlantic. In: NEUMANN,E. R. & RAMBERG,I. B, (eds) Petrology and Geochemistry of Continental Rifts. Reidel, Dordrecht, 173-179. -
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SHOTTON,F. W. 1956. Some aspects of the New Red Desert in Britain. Liverpool and Manchester Geological Journal, 1, 45(b465. SMITH, D. B. 1968. The Hampole Beds--a significant marker in the Lower Magnesian Limestone of Yorkshire, Derbyshire and Nottinghamshire. Proceedings of the Yorkshire Geological Society, 36, 463-477. 1970a. The palaeogeography of the British Zechstein. In: RAU, J. L. & DELLWlG, L. (eds) Third Symposium on Salt. Northern Ohio Geological Society, 20-23. -1970b. Permian & Trias. Transactions of the Natural History Society of Northumberland, 41, 66-91. 1971. Possible displacive halite in the Permian Upper Evaporite Group of northeast Yorkshire. Sedimentology, 17, 221-232. 1972. The Lower Permian in the British Isles. In: FALKE, H. (ed.) Rotliegend, essays on European Lower Permian. Brill, 1-33. 1973. The origin of the Permian Middle and Upper Potash deposits of Yorkshire: an alternative hypothesis. Proceedings of the Yorkshire Geological Society, 39, 327-346. -1974a. The Permian. In: RAYNER,D. H. & HEMINGWAY,J. E. (eds) The Geology and Mineral Resources of Yorkshire. Yorkshire Geological Society, 115-144. 1974b. The stratigraphy and sedimentology of Permian rocks in north Yorkshire. Journal of Earth Sciences, 8, 365-386. 1980. The evolution of the English Zechstein Basin. Contributions to Sedimentology, 9, 7-34. 1986. The Trow Point Bed. In: HARWOOD, G. M. & SMITH, D. B. (eds) The English Zechstein and Related Topics. Geological Society, London, Special Publication, 22, 113-125. 1989. The palaeogeography of the English Zechstein. Proceedings of the Yorkshire Geological Society. , BRUNSTROM,R. G. W., MANNING,P. I., SIMPSON,S. ~,t SHOTTON,F. W. 1974. A Correlation of the Permian Rocks in the British Isles. Geological Society, London, Special Report, 5. -& CROSBY,A. 1979. The regional stratigraphicai context of Zechstein 3 and 4 potash deposits in the British sector of the southern North Sea. Economic Geology, 74, 397-408. - - , HARWOOD,G. M., PATTISON,J. & PETTIGREW,T. 1986. A revised nomenclature for Upper Permian strata in north-east England. In: HARWOOD,G. M. & SMITH, D. B. (eds) The English Zechstein and Related Topics. Geological Society, London, Special Publication, 22, 9-17. - - & TAYLOR,J. C. M. 1989. A north-west passage to the southern Zechstein Basin of the UK North Sea. Proceedings of the Yorkshire Geological Society, 47, 313320. STEELE, R. P. 1981. Aeolian Sands and Sandstones. PhD thesis, University of Durham. STEWART,F. H. 1949. The Petrology of the evaporites of Eskdale No. 2 boring, east Yorkshire. Part I, The Lower Evaporite Bed. Mineralogical Magazine, 28, 621675. 1951a. Part II, The Middle Evaporite Bed. Mineralogical Magazine, 29, 445475. 1951b. Part III, The Upper Evaporite Bed. Mineralogical Magazine, 29, 557572. 1953. Early gypsum in the Permian evaporites of north-eastern England. Proceedings of the Geologists' Association, 64, 33-39. 1956. Replacements involving early carnallite in the potassium-bearing evaporites of Yorkshire. Mineralogical Magazine, 31, 127-135. 1963. The Permian Lower Evaporites of Fordon in Yorkshire. Proceedings of the Yorkshire Geological Society, 34, 1-44. TATE, M. P. & DOBSON, M. R. 1989. Late Permian to early Mesozoic rifting and sedimentation offshore N.W. Ireland. Marine and Petroleum Geology, 6, 49-59. TAYLOR, J. C. M. 1980. Origin of the Werraanhydrit of the U.K. Southern North Sea--a reappraisal. Contributions to Sedimentology, 9, 91-113. 1984. Late Permian-Zechstein. In: GLENNIE, K. W. (ed.) Introduction to the Petroleum Geology of the North Sea. Blackwell, Oxford, 61-83. ibid. 2nd edition, 87-107. & COLaXR, V. S. 1975. Zechstein of the British sector of the southern North Sea Basin. In: WOODLAND,A. W. (ed.) Petroleum Geology of the Continental Shelf of North-West Europe. Applied Science, Barking, 249-263. WILLS, L. J. 1948. The Palaeogeography of the Midlands. Hodder & Stoughton, London. -1973. A palaeogeographical map of the Palaeozoic floor below the Upper Permian and Mesozoic formations in England and Wales. Geological Society, London, Memoir, 7.
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Triassic G. W A R R I N G T O N
& H.
"The Triassic rocks of Great Britain, if one may judge from statements in the t e x t books, are among the simplest of our deposits; but a closer inspection reveals a number of very difficult and intensely interesting problems hidden behind this apparent simplicity." L. J. Wills, 1910.
C. I V I M E Y - C O O K
time sense, that vary inversely with the amount of stratigraphic control available--both within individual maps and between successive ones. GW JURASSIC
During latest Permian times, northerly-sourced epeiric marine environments, in which the Zechstein sequence formed, were replaced by predominantly clastic sedimentary regimes. The succeeding Triassic deposits, representing about 45 million years, are largely continental; some formed under transient marine influences but few are biogenic. They succeed Permian deposits conformably and overstep these to rest unconformably upon older rocks. Over much of the region the youngest Triassic deposits represent a transgression that re-established marine environments before the end of the Period. The region lay in Laurasia, the northern part of the Pangaean supercontinent, some 15 to 20 ~ north of the equator and close to the northern margin of the Tethys (Tollmann & Kristan-Tollmann 1985) but separated from that ocean by the Variscan mountain chain, denudation of which continued from Permian into Triassic times. A monsoonal climate is envisaged (Robinson 1973; Parrish & Curtis 1982; Parrish et al. 1982). The principal onshore Triassic occurrences are in England and Northern ireland; others in the British Isles are relatively minor (Warrington et al. 1980, figs 2 & 3). Concealed developments in The Netherlands (Map Tr2) and northeast France (Mrgnien & Mrgnien 1980; Debrand-Passard 1980; Berners et al. 1984; Maps Tr2-3b & 4c) are contiguous with the tripartite German sequence from which the System name derives (Alberfi 1834). Triassic and presumed Triassic deposits occur extensively offshore (Geiger & Hopping 1968; Rhys 1974; Deegan & Scull 1977; Ziegler 1982; Naylor & Shannon 1982; Vollset & Dor6 1984; Lervik et al. 1989; Fisher & Mudge 1990; Steel & Ryseth 1990; see also Woodland 1975; Illing & Hobson 1981; Brooks et al. 1986; Brooks & Glennie 1987; Ziegler 1987a). Biostratigraphic evidence is of variable nature and quality (Warrington 1976, 1986), and is lacking for many sequences. The marine faunas upon which Triassic chronostratigraphic divisions are based (Tozer 1984) are virtually absent. Macrofossils indicate diverse biota but are scarce, reflecting preservational factors, and are facies-related; some provide environmental or dating evidence. Plant spores and pollen (miospores) are stratigraphically and geographically more widespread, and permit correlation regionally across facies boundaries and with the international stages. Other microfossils occur less widely but some are important for dating (conodonts)and environmental interpretation (marine phytoplankton). During the Triassic tensional crustal stresses occurred around sites where later continental rifting resulted in the opening of the North Atlantic ocean (Warrington 1970, p. 221). The stresses led to widespread growth faulting and rift development (Whittaker 1985; Ziegler 1987b and in Manspeizer 1988) which influenced the distribution and character of the deposits formed. Thickness changes are commonlY imaged in seismic profiles across faults but dating the implied movements may be hampered by poor biostratigraphic control. Maps Trla-4a include a framework of structures that m a y have been active in Triassic times though movement on any one at a particular time or throughout the period is not implicit. Major listric faults, some resulting from tensional reactivation of compressional (thrust) structures, define basins in southern areas (Chadwick et al. 1983; Whittaker 1985; Lake & Karner 1987; Cheadle et al. 1987; Penn et al. 1987; Chapman 1989) and to the north of Scotland (Kirton & Hitchen 1987). Some basin infills show a 'steers head' profile, indicating cessation of contemporaneous faulting in the later Triassic (Whittaker 1985), Late Triassic depositional margins are preserved in South Wales, the Mendips, Charnwood Forest and at subcrop around the London Platform. Elsewhere the limits shown are conjectural and the non-depositional areas depicted may be maxima. The present distribution of Triassic deposits may differ little from that originally defined by syndepositional faulting in many areas. Deformation and erosion has occurred over and around halokinetic structures and other sites of uplift in the North Sea and possibly elsewhere. The maps are based primarily upon the better-known successions of England and the southern North Sea in which five 'associations' (Fig. 1), based upon broad characters of lithofacies and depositional regime, are recognized and used for descriptive purposes. Biostratigraphic evidence and regional lithostratigraphic markers constrain the successive reconstructions of those areas. Other biostratigraphically controlled successions" are integrated into the appropriate reconstruction; where such evidence is lacking the interpretation presented is compatible in the regional geographical and stratigraphical context. Though related as closely as possible to the chronostratigraphic divisions of the Period, the maps have 'depths of focus', in a 9 1992The GeologicalSociety
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Fig. 1. Triassic lithofacies in England and the southern North Sea after Warrington et al. 1980) with British onshore nomenclature and the facies associations adopted in this account: Chronostratigraphy after Zapfe (1983), with "Rhaetian" as advocated by the lUGS Subcommission on Triassic Stratigraphy (Wiedmann et al. 1979); geochronology after Forster & Warrington (1985).
T r l a : E a r l y to mid S c y t h i a n The map portrays A s s o c i a t i o n A , which comprises generally unfossiliferous clastic sequences, largely of continental origin, below the Hardegsen Disconformity (Trusheim 1963) which correlates with a level in the higher part of the German Middle Bunter. It includes parts of the Bacton Group (southern North Sea) and Sherwood Sandstone Group (Britain), and possibly some beds below the Sherwood Sandstone Group, and spans some five million years. Much of the Early Triassic or Scythian, a series comprising up to four stages (Zapfe 1983; Zakharov 1987), is represented by this association (Fig. 1). The base of the Otoceras w o o d w a r d i Zone, defining the Permian-Triassic boundary (Tozer 1988), is not identified in the British Isles where the boundary can be positioned only approximately between occurrences of late Permian fossils and ones of late Scythian or younger Triassic age. At Kingscourt, Ireland (Visscher 1971), a late Permian microflora is succeeded, within 30 to 60 m, by one comparable with those known from earliest Triassic beds, with O t o c e r a s faunas, in east Greenland (Balme 1980; Piasecki 1984); elsewhere biostratigraphic control on the position of the System boundary is much poorer. Palynomorphs of Dienerian and Smithian age occur at unspecified North Sea sites (Fisher 1979; Fisher & Mudge 1990) in, respectively, possible equivalents of the Volpriehausen and Hardegsen divisions of the German Middle Bunter, below the Hardegsen Disconformity. More typically, the lowest occurrences of Triassic fossils are above that level, in Association B. In Devon, at least 650 m of beds without biostratigraphic evidence of age separate a horizon with late Permian fossils (Warrington & Scrivener 1990) from the lowest occurrence of Triassic fossils, which comprises Middle Triassic vertebrates. In Northern Ireland, the East Irish Sea Basin, northwest, central and northeast England, thick unfossiliferous sequences also separate the levels of occurrence of late Permian fossils from the lowest of Triassic age, the most widespread of which are late Scythian and Anisian microfloras (Pattison et al. 1973; Warrington et al. 1980; Jackson et al. 1987). In the virtual absence of biostratigraphic control, a conglomerate unit has been used as a datum for the construction of Map Trla. This unit overlies fluvial and aeolian deposits that, around the East Irish Sea and North Sea basins, have a diachronous interface with underlying argillaceous beds, reflecting the spread of coarser clastic sediments basinwards across finer distal deposits that formed in extensive playas towards the end of the Permian (Pattison et al. 1973; Warrington 1974a; Warrington et al. 1980; Smith 1989). It is within such sequences, and below the conglomeratic unit 97
98
TRIASSIC
and its presumed lateral equivalents, that the Permian-Triassic boundary is arbitrarily placed (Warrington et al. 1980). The conglomeratic unit is thus envisaged as slightly younger than that boundary and may be Dienerian to Smithian, the age of palynomorphs from presumed equivalent beds in the North Sea. The conglomeratic unit includes the Budleigh Salterton Pebble Beds of southwest England, and the Kidderminster, Nottingham Castle and Chester Pebble Beds formations of central England (Warrington et al. 1980). These comprise coarse clastic sediments with exotic extraclasts, many regarded as having a southerly (Armorican) provenance (Wills 1950; Fitch et al. 1966). Palaeocurrent vectors and a decrease in the size and abundance of extraclasts from southern and central England into the East Irish Sea and North Sea basins indicate northward transport. The conglomerates formed in fluvial braided stream environments (Steel & Thompson 1983; Smith 1990) and largely comprise channel deposits; they pass laterally northwest- and northeastwards into dominantly arenaceous sequences, such as the Bunter Sandstone Formation of the southern North Sea, that represent more distal fluvial environments. To the south, catchment areas in the Variscan mountain chain, between the map area and the Tethys ocean, probably received high but very seasonal rainfall under a monsoonal regime of alternating low (summer) and high (winter) pressure systems (Parrish & Curtis 1982; Parrish et al. 1982). Rivers draining northwards into the region probably had peak discharge during the summer season when southeasterly winds prevailed (Clemmensen 1978). Discharge waned during the succeeding drier months when the size and number of active channels probably declined and aeolian deposition may have occurred locally under the influence of seasonal northeasterly winds. The Hopeman Sandstone Formation in the Moray Firth Basin, though possibly latest Permian (Benton & Walker 1985), comprises dune sediments, including remains of complex star dunes (Clemmensen 1987), that reflect the prevalence of such winds around this time. Karst development with caves that were infilled in the late Triassic may have begun in early Triassic or even Permian times, when Carboniferous and older limestones were exposed above the water table. In the northern North Sea correlative sequences probably form the lowest part of the Skagerrak and Smith Bank formations, of proximal and distal continental character respectively (Lervik et al. 1989). Sequences imaged on seismic sections from westerly areas may include correlatives but where drilled these lack biostratigraphical evidence (Hitchen & Ritchie 1987; Robeson et al. 1988; Tate & Dobson 1989). Deposits in fault-bounded basins to the northwest of Ireland and mainland Scotland are of continental facies as, probably, are inferred occurrences in the Rockall and Faeroe troughs. The sedimentary models of Steel (1977), Steel & Wilson (1975), Clemmensen et aL (1980) and Frostick & Reid (1987) may be applicable to those sequences which, because of lack of biostratigraphic control, are also shown on Maps Trlb to Tr4a. Sporadic macrofossils indicate mainly continental environments, though probably more populous than the scarcity of remains suggests. The scarce microfloras reflect a vegetation including lycopsids and gymnosperms, principally conifers. Trace fossils include vertebrate tracks, indicating subaerial conditions and reflecting, in central England, a varied terrestrial fauna (Sarjeant 1974). Fish remains there, and in Northern Ireland, indicate aqueous environments, the salinity of which is unclear. Foraminifera, acritarchs and tintinnids are associated with early Triassic microfloras at Kingscourt (Visscher 1971) and acritarchs occur in possible early Triassic deposits in the Central Graben (Brennand 1975). If in situ these indicate marine conditions which appear anomalous in a context of regional continental sedimentation. The source of the Kingscourt occurrence may lie west of Ireland where presumed Permo-Triassic deposits, including interpreted marine facies, occur in the Porcupine Trough (Tate & Dobson 1989); that of the Central Graben occurrences is obscure. East to west growth faults influenced development of the Wessex Basin (Allen & Holloway 1984; Chadwick et al. 1983; Whittaker 1985; Lake & Karner 1987), and similar structures probably affected areas farther west (Tucker & Arter 1987; van Hoorn 1987; Ziegler 1987b; Chapman 1989) and south (Penn et aL 1987). In central England deposition was confined, by faulting, to a narrow north-south corridor (Chadwick 1985; Chadwick & Smith 1988). The Hardegsen Disconformity resulted from earth movements, followed by erosion, that affected northwest Europe during deposition of the Hardegsen division of the German Middle Bunter (Trusheim 1963). In Britain this activity is reflected in the disconformity between Association A and the succeeding Association B, a more fossiliferous, heterogeneous unit with significant input from marine sources. In southern England the conglomerate unit (Map Trla) is believed to have been eroded from central and northern parts of the Wessex Basin after the Hardegsen movements (Holloway et al. 1989). In the west of the basin it is overlain disconformably by Middle Triassic deposits but in central England other units separate the equivalent beds. GW
Trlb, Tr2, Tr3a: Earliest Anisian to earliest Carnian The maps portray A s s o c i a t i o n B, which comprises a complex of basinal, argillaceous, evaporite-bearing deposits and laterally equivalent coarser clastic sequences representing marine-sourced and continental environments respectively. It spans some 20 million years and includes post-Hardegsen beds in the Sherwood Sandstone and Bacton groups and those parts of the
Mercia Mudstone and Haisborough groups below the level of the Arden Sandstone Member and its correlatives (Association C, Fig. 1). After the Hardegsen movements the southern North Sea Basin was temporarily separated from those farther west (Map Trlb). The basinal facies in the association includes thick halite deposits (Trlb, Tr2, Tr3a) and a calcareous facies, equivalent to the German Muschelkalk (Geiger & Hopping 1968), occurs in the southern North Sea; however, largely continental deposition persisted north of present latitude 56~ Fossils are more abundant than in Association A; whilst macrofossils are largely restricted to the coarser clastic facies (Maps Trlb, Tr2), land-derived microfloras occur in the basinal facies also and afford wider regional biostratigraphic control. Halite deposits occur at three main levels in the southern North Sea sequence and those farther west in Britain; these are dated, largely by palynology, as earliest Anisian, late Anisian, and latest Ladinian (?) to early Carnian, and are used as datum levels for Maps Trlb, Tr2 and Tr3a. In south and west Britain the Otter Sandstone, Bromsgrove Sandstone and Helsby Sandstone formations (Warrington et aL 1980) represent continental fluvial environments with northward-flowing rivers (Map Trlb). They comprise a complex of fining-upward sedimentary cycles that represent deposition initially in predominantly braided stream environments and later in more mature meandering river systems. Deposits of the latter environment, which incorporate more vertical accretion (overbank) sediments, are more characteristic of the distal (northerly) and younger parts of the fluvial succession, and diminish in importance southwards (Warrington 1970); river discharge remained markedly seasonal. In the Cheshire Basin aeolian deposits interspersed with fluvial sequences (Thompson 1969, 1970) record northeasterly winds that probably prevailed during drier winter seasons (Map Trlb); subaerial conditions are also indicated there and elsewhere by vertebrate tracks (Sarjeant 1974). The fluvial facies passes distally into interbedded sandstones, siltstones and mudstones representing fresh to brackish water, estuarine and intertidal environments that bordered areas in which evaporite-bearing mudstones of the Mercia Mudstone Group accumulated farther northwest. In that Group, fine-grained clastic units, containing sulphates (gypsum and anhydrite) and dolomite, are interspersed with thick halites that are interbedded with varying amounts of mudstone (e.g. Wilson 1990) and pass laterally into sulphate-rich mudstone sequences. The clastics comprise fine terrigenous sediment deposited partly under subaerial and partly under subaqueous conditions (Arthurton 1980). The water bodies, in which some clays and evaporites formed (Jeans 1978), were shallow, with little current or tidal disturbance. Connection with a marine source is indicated by geochemistry (Holser & Wilgus 1981), clay mineralogy (Jeans 1978) and fossils (Warrington 1974b, 1981) though salinity would vary with position in a basin and the relative rates of fluvial discharge, marine influx and evaporation. Halite was probably precipitated in shallow bodies of hypersaline water undergoing evaporation with steady replenishment from a marine source (the Tethys ocean), rather than by evaporation of deep isolated water bodies (Warrington 1974b). Sulphates in laterally equivalent mudstones were deposited from interstitial brines in sabkha environments. Analogous facies relations occur in the southern North Sea and Northern Ireland, where fluvial deposits are less extensive, and possibly offshore to the south and west where drilled sequences and biostratigraphic control are scarce (Warrington 1983; Bennet et al. 1985). Contemporaneous faulting influenced the location and nature of Association B deposits; in particular, halites may reflect the geographical and temporal incidence of such activity (Warrington 1974b). Thickness variations are greatest where halite is present in the evaporite-bearing facies (Lott et al. 1982); syndepositional faulting probably aided and localized the precipitation and rapid accumulation of thick halite bodies. The depositional areas probably expanded progressively, as is shown by the re-establishment of continuity between the southern North Sea and more westerly basins, severed during the Hardegsen movements, and by the probable later connection of the central English Channel basins with those farther west, across the Start-Cotentin Ridge. During the Anisian, fine-grained basinal evaporitebearing facies transgressed over areas of former contemporaneous fluvial sedimentation and the transitional and fluvial facies belts migrated diachronously into proximal and basin margin areas (Maps T r l b & Tr2) until the fine-grained facies became geographically dominant (Map Tr3a). Associated with this change and the decrease in coarser clastic sedimentation, was a reduction of the relief of the non-depositionai areas which may have been largely peneplained by mid-Carnian times. Halites are most widespread in the southern North Sea Basin where the earliest Anisian R6tsalinar (Map Trlb) extends into eastern England (Riddler 1981). The Muschelkalk and Keuper Halite members (Maps Tr2 and Tr3a) are more restricted (Brennand 1975). The Rftsalinar and the Muschelkalk Halite occur within the Dowsing Dolomite Formation (Rhys 1974) which includes a dolomitic facies that extends to eastern England and represents the maximum westerly extension of the epicontinental marine Muschelkalk facies of Germany. Farther west the oldest halite is restricted to Northern Ireland and the East Irish Sea Basin (Map Trlb). Late Anisian halite deposits there extend south into the Cheshire Basin (Map Tr2) and, on palynological evidence, include the thick Larne Halite in Northern Ireland, formerly assigned a late Triassic age (Warrington e t al. 1980). The youngest halite is represented in the East Irish Sea Basin and in central and southern England. Deposition of this unit may have begun in latest Ladinian times though palynological evidence shows it to be largely of early Carnian age;
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Map Tr3a depicts the earlier part of this time span. Halites in the St George's Channel and Bristol Channel basins (Barr et aL 1981; Kamerling 1979; Penn 1987) are interpreted as correlatives (Map Tr3a) but biostratigraphic evidence is lacking; halite dated palynologically as Carnian (Bennet et al. 1985) is present in the South Western Approaches (Map Tr3a). Undated carbonates in the North Celtic Sea and Fastnet basins (Colin et al. 1981; Robinson et al. 1981), and a coastal microfauna in the latter (Robinson et al. 1981), may reflect marine-sourced environments. They are depicted (Map Trl b) as early in the deposition of Association B but may be younger. Continental clastic sedimentation, represented by the Teist, Lomvi and lower part of the Lunde formations and equivalent beds in the Smith Bank and Skagerrak formations (Lervik et al. 1989), characterized most parts of the central and northern North Sea. Marine-sourced environments extended briefly northwards to the Egersund Sub-basin (Map Tr2; Vollset & Dor6 1984). Tufts low in the Lunde Formation in the Statfjord Field area (Vollset & Dor6 1984) are shown in Map Tr3a but may be slightly younger. West of Scotland and south of Ireland biostratigraphic evidence is generally lacking. Relationships depicted between Association B and the successions there are largely conjectural. Undated volcaniclastics northwest of Ireland (Robeson et al. 1988; Tare & Dobson 1989) and in the north -Celtic Sea Basin (Colin et al. 1981) are shown on Map Tr3a but may be younger. The source of these widely separated offshore occurrences of volcaniclastics (Map Tr3a) is not known though late Triassic lavas occur in the Aquitaine Basin, France (Strvaux & Winnock 1974; Boury et al. 1977). In southern Norway dykes that possibly fed vents were intruded mainly in late Triassic times (c. 220 Ma, Faerseth et al. 1976) but the interpretation of lavas in red-beds in the North Sea Forties Field area as possibly Triassic (Woodhall & Knox 1979), implying a vent in that area, has been challenged (Ritchie et al. 1988). The volcanics may be associated with the earliest (c. 225 Ma, Carnian) of three magmatic phases that Harrison et al. (1979) regarded as probably related to North Atlantic continental rifting. Base metal deposits in the Triassic in central England may also be related; they are considered epigenetic and were possibly precipitated from intrastratal brines in structural traps shared with hydrocarbon accumulations (Warrington 1980). The principal host rocks, in the Sherwood Sandstone, are some 15 million years older than the Carnian magmatic phase but epigenetic activity then may have had surface expression in hot springs (cf. Ford 1969). Macrofossils in fluvial facies in south and central England reflect a varied flora and fauna (Wills 1910, 1947; Walker 1969; Paton 1974; Milner et al. 1990) and are commonest in deposits representing meandering river systems. The flora included equisetalean plants, which probably inhabited damp floodplain areas, and conifers adapted to drier habitats. Microfloral remains reflect a vegetation including lycopsids, sphenopsids and pteropsids, but dominated by gymnosperms, principally conifers though with cycadalean or ginkgoalean components also. Invertebrates include crustaceans (Euestheria) indicative of fresh to brackish water environments in seasonal pools, and scorpions, suggestive of dry terrestrial areas which were probably 'enlivened by their chirping or hissing' (Wills 1947). Sporadic fish remains include dipnoi, indicative of river systems prone to seasonal drought. Vertebrate tracks (Sarjeant 1974) and bioturbation are widespread, and land vertebrates are also represented by remains of labyrinthodont amphibia and reptiles, including the herbivorous rhynchosaurs (Benton 1984, 1990). Similar assemblages persist into the transitional facies from which intertidal trace fossil associations (Pollard & Steel 1978; Ireland et al. 1978; Pollard 1981) and also bivalves, the brachiopod Lingula, ostracods and acritarchs have been reported (Rose & Kent 1955; Ivimey-Cook 1978; Warrington 1981) and afford evidence of the marine affinity of the basinal environments into which the fluvial systems discharged. The basinal facies is generally largely devoid of fossils other than land-derived miospores. Sporadic Euestheria may be allochthonous, and a solitary plecopteran insect wing (Thompson 1966) affords no evidence of the nature of the depositional environment. However, acritarchs, indicating connection to a marine source, occur widely in this facies in small numbers (Warrington 1981). GW
Tr4b: Late Carnian Map Tr4b illustrates Association C, which comprises deltaic and estuarine deposits of a brief phase, between those of marine-sourced evaporitic sedimentation in Associations B and D, when more open connection existed to the Tethys ocean. It comprises the Arden Sandstone Member and correlatives in the Mercia Mudstone Group in England (Fig. 1; Map Tr4b) and in the Haisborough Group of the southern North Sea. The association is of late Carnian (Tuvalian) age (c. 222 Ma). Though relatively thin, it contrasts with the contiguous largely red-brown mudstone facies by being comparatively fossiliferous (Map Tr4b), dominantly grey-green, and including siltstones and, locally, thick sandstones. It is comparable with the German Schiifsandstein (Wurster 1964). Dominantly arenaceous sequences and thinner, more argillaceous, laterally equivalent sequences are interpreted, respectively, as deposits of distributary channels and interdistributary areas in a deltaic to estuarine environment (Warrington & Williams 1984). Subaerial conditions are indicated locally by vertebrate tracks and possible insect burrows. Most of the sediment accumulated in water ranging from fresh or brackish towards marine in character,
and displays much slumping, thixotropic disturbance and bioturbation, principally of Planolites-type. In contrast with Associations A and B, which display marked thickness variations related to contemporaneous faulting, Association C is comparatively uniformly developed over a wide area, indicating the decline or cessation of such activity during Carnian times. Improved connection with the marine Tethys province is reflected in the biota and probably resulted from regional isostatic lowering of southerly areas following the more localized tensional--extensional movements of earlier Triassic times. Consequent drainage or the rejuvenation of older systems initiated by such changes in relative levels resulted in the transport of coarser debris that had accumulated on largely peneplained areas and its redeposition in distributary channels spreading over a littoral or near-littoral surface. East- to southeasterly current flows recorded in England (Map Tr4b) suggest consequent drainage from relatively higher areas to the west and northwest. Conditions in the distal parts of the distributary channels varied from freshwater to near-marine, with water of marine origin penetrating into the channel system tidally or when fluvial discharge waned. The topographic changes indicated may have influenced climate and temporarily enhanced precipitation; climatic change alone (Simms & Ruffell 1990) is unlikely to have produced Association C. The Association is not known in the northern North Sea, northern England or areas farther northwest; any correlatives in those areas are in continental facies. Terrestrial components of the biota (Map Tr4b) comprise equisetalean and coniferalean plants, and the tracks and remains of amphibians and reptiles. Microfloras reflect a vegetation including sphenopsids and pteropsids, but dominated by gymnosperms, mainly conifers but including cycadalean or ginkgoalean types. Bryophyte spores (Porcellispora) testify to damp terrestrial habitats that may also have been colonized by equisetalean plants and ferns. Water-laid sediments are extensively bioturbated. A green alga (Plaesiodictyon) regarded as associated with brackish-water environments (Wille 1970) is widespread, and crustaceans (Euestheria) indicative of fresh to brackish-water conditions in seasonal pools are common. Sporadic dipnoan remains suggest a river system prone to seasonal drought. Marine connection is indicated by sporadic bivalves (Map Tr4b) and widely occurring remains, mostly teeth, of hybodont and xenacanthiform sharks. GW
Tr3b: Latest Norian to earliest Rhaetian The map illustrates Association D, which comprises basinal argillaceous evaporite-bearing deposits and laterally equivalent coarser clastic sequences in the upper part of the Mercia Mudstone Group in Britain (Fig. I) and equivalent beds in the Triton Anhydritic Formation at the top of the Haisborough Group in the southern North Sea; this sequence spans some 12 million years, from about the end of the Carnian into Rhaetian times. It consists largely of red mudstones and represents a reversion from Association C conditions to ones broadly similar to those of the upper part of Association B. Coarse clastic deposits in basin-margin situations are of local origin and limited extent; they pass laterally into finer-grained basinal sediments within a short distance. The Doiomitic Conglomerate, which locally marks depositional margins in southwestern Britain, formed as subaerial screes and in fluvial environments (Ivimey-Cook 1974; Tucker 1977, 1978). The basinal fine-grained clastic facies includes terrigenous sediment that accumulated in both subaerial and subaqueous environments. Gradual thickness changes suggest that deposition occurred in broad, gently subsiding basins, with little contemporaneous faulting, a continuation of the quiet tectonic conditions of Association C times. Only sulphate evaporites are present, in contrast with similar facies in Association B which also contain halite. They form extensive concentrations deposited from interstitial brines or shallow hypersaline water bodies in sabkha environments. The brines were partly marine and partly continental in origin (Taylor 1983). The sabkhas may be envisaged as largely supratidal and bordering hypersaline marine-sourced epeiric water bodies. The highest beds comprise the Blue Anchor Formation (Warrington et al. 1980; Mayall 1981; Warrington & Whittaker 1984) which contrasts with much of the Association by being grey-green in colour. This formation is usually succeeded unconformably, above an eroded and bored surface, by the Westbury Formation (Penarth Group, Association E; Fig. 1). This unconformity is less marked in southwest Britain where the earlier formation is thicker, lithologically heterogeneous in its upper part, and broadly transitional from the underlying redbrown mudstones to the succeeding Westbury Formation. The sedimentology and biota of this transition record a change from supratidal sabkhas, through intertidal sabkhas to a shallow marine environment that marks the commencement of the Rhaetian transgression which established marine conditions through much of Britain. Farther north supratidal sabkha and playa environments predominated. The deposits overstep onto former nondepositional areas in several parts of Britain, and some infill fissures and caves in Carboniferous Limestone outcrops. Continental clastic sedimentation characterized most parts of the central and northern North Sea where correlatives of Association D occur in the Lunde Formation and equivalent beds in the Smith Bank and Skagerrak formations (Lervik et al. 1989). West of Scotland biostratigraphic evidence is lacking. Rhaetian deposits overlie a sequence with volcaniclastics northwest of Ireland (Robeson et al. 1988; Tate & Dobson 1989) but the volcaniclastics (Map Tr3b) are undated and may be older than depicted.
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The Association is largely unfossiliferous, though rich and diverse biota are present locally. Reptiles in continental deposits at Lossiemouth in the Moray Firth Basin may be early Norian; they include two herbivores, four small omnivores and two carnivores (Benton & Walker 1985). Crustacea (Euestheria), indicative of fresh to brackish water conditions in ephemeral pools, occur sporadically in the Blue Anchor Formation in Northern Ireland and central England, and isolated fish scales and miospores occur in the latter area. In South Wales, land vertebrate tracks (Tucker & Burchette 1977) signify subaerial conditions in marginal depositional areas, and algal limestones, almost the only biogenic deposits in the British Trias, indicate local fresh to brackish water conditions (Ivimey-Cook 1974). The lower beds of the Blue Anchor Formation in southwest Britain contain only miospores and trace fossils. An ichnofauna from higher beds (Mayall 1981) includes Diplocraterion and suggests intertidal environments. Dinoflagellate cysts, the earliest known from the region, and acritarchs, foraminifers, molluscs, fish and other remains that appear within the highest beds reflect the inception of marine conditions (Map Tr3b; Warrington 1981; Warrington & Whittaker 1984; Warrington & Ivimey-Cook 1990). Land areas in southwest Britain supported a rich and diverse vertebrate fauna, preserved in sediments infilling karst features on areas of Carboniferous Limestone that were exposed from earlier Triassic or even Permian times. The infills are mostly poorly dated; one, believed to pre-date the Rhaetian marine transgression, contains remains of lepidosaurs, archosaurs, sphenodontids and an insectivorous early mammal (Kuehneotherium) that may have been nocturnal in habit (Fraser et al. 1985). Microfloras are richest in the Blue Anchor Formation and reflect a vegetation including pteridophytes but dominated by gymnosperms, principally cheirolepidiacean conifers. GW
Tr4a, 4c: Mid to late Rhaetian The maps illustrate Association E, which comprises generally argillaceous and calcareous deposits, commonly richly fossiliferous, that formed during the mid- to late Rhaetian marine transgression and in the succeeding marine environments. It includes the Penarth Group and beds in the Lias Group below the lowest occurrence of the ammonite genus Psiloceras which defines the base of the Jurassic in Britain (Cope et al. 1980; Warrington et all 1980). Quite diverse marine faunas are recovered: foraminifers, corals, inarticulate brachiopods (but no articulates), bivalves, gastropods, ostracods, cirripedes, ophiuroids, echinoids, conodonts and vertebrates (Warrington & Ivimey-Cook 1990). A single juvenile psiloceratid ammonite has been found in the Westbury Formation (Donovan et al. 1989) but ammonite taxa as a whole were much reduced in geographical range and variety at this time. Non-marine vertebrates and plants give indications of onshore life; e.g. the massive jawbone of the teratosaurid Zanclodon in Glamorgan, and remains found in karst fissure fills on the Carboniferous Limestone islands in the Severn Estuary region. Faunal and floral diversity seems to have been greater than in the early Hettangian. The transgression started during the deposition of the Blue Anchor Formation in the southwest and spread rapidly northwards, depositing dark marine shales (Westbury Formation, Map Tr4a), largely co-terminously, over low-lying areas occupied by the continental deposits of Association D. The ultimate extent of the transgression is conjectural, little evidence now existing of the environments then present in most parts of Ireland, Scotland and Wales. However, it reached from the southwest, where thick grey claystones underlie 9.5m of limestones (Well 72/10-1a) in the Western Approaches Trough (Bennet et al. 1985), to around present latitude 60~ along several routes. Marine, and local lagoonal and evaporitic sequences, are known from the Penarth Group in the Fastnet and Celtic Sea Basins (Ainsworth et al. 1989). The early deposits of the transgression are well exposed onshore around the Bristol Channel (Ivimey-Cook 1974; Mayall 1981; Whittaker & Green 1983; Waters & Lawrence 1987). Marine environments also extended south of Cornubia, the size of which is very speculative, into the English Channel Basin. Arenaceous deposits appear in eastern Hampshire (Portsdown Borehole, Taitt & Kent 1958). Farther east, alternating red beds, breccias and marine shales dated only as 'Rhaeto-Lias' (Ivimey-Cook in Young & Lake 1988) indicate fluctuating conditions which may have extended southwards into what is now the English Channel. No clear evidence has been found of a marine connection southwards from here through northern France or Belgium. Islands, mainly of Carboniferous Limestone, in Glamorgan, Avon, Somerset and Gloucestershire are surrounded and even permeated by a diversity of deposits. The associated faunas and floras include bones of reptiles and early mammals brought into fissures and caves (Fraser 1988; Fraser & Unwin 1990; Marshall & Whiteside 1980). In Glamorgan rivers flowed south, across what is now the South Wales coalfield, depositing coarse sand in a channel north of the Cowbridge island and forming two massive sand bodies (Francis 1959). To the west, in the Bristol Channel, wells 102/28-1 and 103/18-1 may also be in marginal deposits off the Welsh Island, the northerly extent of which is unclear. West of this island, marginal deposits extend northeastwards into a playa-like environment, with dolomitized calcarenites and carbonates (Mochras borehole, Harrison 1971). A Celtic Sea channel west of Wales led into the Irish Sea. In the Kish Bank Basin (Jenner 1981) Trias is overlain by Lower Jurassic but marine Rhaetian is unproven. Westbury Formation mudstones are however known farther north in northeastern Ireland (Ivimey-Cook 1975), Arran, Mull-where over 10m of marine shale are known (Lee & Pringle 1932)--and
south of Islay (Owens & Marshall 1978), Sandstones with marine fossils at Loch Aline may be of this age or slightly younger; at Applecross, Dr P. E. Kent (unpublished notes) found 2 m of greenish sandstones and shales which may constitute a marginal facies. Farther north the Westbury Formation is unproven; BGS shallow borehole 72/37 recovered calcilutites and calcareous mudstones with ? Rhaeto-Hettangian miospores, and BGS borehole 72/34 encountered red siltstone with sandy lenses (Owens & Marshall 1978). North and northwest of the Hebrides little reliable evidence is available and the palaeogeography (Map Tr4a) has been related to possible structural lines. West of Ireland, the North Porcupine Basin, Erris Trough and Donegal Basin contain dated Rhaetian sediments indicating complex coastal, including marine, environments associated with the transgression and succeeded by Hettangian deposits (Tare & Dobson 1989). In central England shallowing or current sorting produced local thinner, more sandy sequences; the erosion of local islands is exemplified b y Charnian pebbles in the basal 'bone-bed' near Leicester (Harrison 1876). Shoreline conditions are proved along the northwestern flank of the AngloBrabant landmass. The arenaceous Twyford Beds (Donovan et al. 1979; Horton et al. 1987) extend northeast, fairly continuously, from near Oxford to Norfolk. Their easterly continuation is less clear although they may occur in the North Creake borehole (Kent 1947) and link with the sand- and siltrich sequences of the southern North Sea. The Westbury Formation thickens and becomes more silty eastwards in Lincolnshire and Yorkshire. Offshore, especially east of the Dowsing Fault, it includes substantial beds of sand with only thin shale interbeds, as in well 49/21-2, the type section for the Winterton Formation (Rhys 1974); Lott & Warrington (1988) reviewed these beds and rejected this latter term. The northwards extent of the marine facies in the North Sea is conjectural. It may reach to between 57~ and 58~ (present latitude) and penetrate westwards, towards the Inner Moray Firth Basin. Here alluvial and lagoonal red-brown shales of Rhaetian-Hettangian age occur in the Beatrice Field (Linsley et al. 1980), distally to the Dunrobin Pier sandstones and conglomerate (see also Andrews et al. 1990). In the northeast, rifting continued in the Viking Graben and East Shetland Basin where continental sedimentation persisted. Here, Lunde Formation lacustrine and fluvial deposits are succeeded by the upward coarsening Statfjord Formation, comprising deposits of distal fluvial fans and coastal environments, including red shales, silty dolomitic limestones and sandstones overlain by coarser sandstones and early Jurassic marine sediments (Roe & Steel 1985; see also Steel & Ryseth 1990). The Westbury Formation is succeeded by the calcareous beds of the Lilstock Formation, comprising the Cotham Member and overlying Langport Member (Map Tr4c). The fine-grained, finely micaceous, grey-green mudstones and fine white silts of the Cotham Member are very widespread in mainland Britain; sequences range from very thin in the southwest to over 9 m in eastern Lincolnshire. The marine fauna of the earliest beds rapidly dies out upwards. Algal knoll reefs (Cotham Marble: Hamilton 1961), a desiccation level filled with coarser sediments (including ooids) in the eastern Bristol Channel area (Whittaker & Green 1983; Ivimey-Cook 1974), and slumps (Mayall 1983), also occur in these shallow water deposits. Beds with marine phytoplankton (Warrington 1978) alternate with ones containing fish scales (Pholidophorus), Euestheria, darwinulid ostracods and plants such as Naiadita. The facies is widespread, but its sparse fauna and argillaceous lithology (with swelling micas at some localities) suggest deposition in a very extensive lagoonal environment in which organisms responded to periodic incursions of fresh water alternating with more saline and even hypersaline conditions. The redistribution of periodic ashfalls could account for some it its unusual lithologicai and biogeographical features. The member is inadequately proven in Kent and Sussex but equivalent beds are known in France and farther east. Reddish/chocolate coloured calcareous mudstones in Lincolnshire were attributed (Kent 1970) to the erosion of Triassic sediments over rising salt diapirs in the North Sea; similar beds occur in Antrim. The member thickens into the North Sea at the expense of the overlying Langport Member and probably also of the earliest beds of the Lower Lias. A prominent constituent off the Langport Member is the 'White Lias', commonly a massively bedded, off-white, very pure calcilutitic limestone, over 5 m thick in southern England and with a characteristic geophysical signature (Whittaker et al. 1985). Locally, as in Devon, it shows traces of a very shallow water environment (Hallam 1960). Littoral conglomeratic developments are found around the islands of Carboniferous rocks in the area of the Severn Estuary. In the Midlands it passes laterally, westwards and northwards, into alternating thin limestones, calcareous silts and mudstones, and eastwards into the calcareous sandy facies (Twyford Beds) fringing the Anglo-Brabant Landmass. In eastern England, from Nottingham northward, only traces of the limestone-dominated facies of the Langport Member are known. In southern England and South Wales the Langport Member is overlain by the 'Ostrea Beds', a sequence of paper shales, mudstones and thin limestones in the basal Lias. However, where this distinctive facies is not developed, as in the Worcester and Cheshire basins and in Northern Ireland, distinguishing a less limestone-dominated Langport Member from the succeeding Lower Lias is subjective. The record of the conodont Misikella posthernsteini (Kozur & Mock) from the lowest beds of the Lias Group in Nottinghamshire (Swift 1989) gives a direct biostratigraphic link with the latest Triassic deposits in Tethyan marine sequences. In the southeast Midlands erosion of the Langport Member and the
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lowest Lias Group beds occurred in latest Triassic to early Jurassic times. The area of deposition of the basal Jurassic planorbis Zone was less extensive than that of the Rhaetian deposits (Donovan et al. 1979). HCI-C
References Papers not listed here are cited in Warrington et al. (1980). AINSWORTH,N. R., O'NEILL, M. & RUTHERFORD,M. M. 1989. Jurassic and Upper Triassic biostratigraphy of the North Celtic Sea and Fastnet Basins. In: BATTEN, D. J. & KEEN, M. C. (eds). Northwest European Micropalaeontology and Palynology, British Micropalaeontological Society/Ellis Horwood, Chichester, 1-44. ALLEN, D. J. & HOLLOWAY,S. 1984. Investigation of the geothermal potential of the UK: The Wessex Basin. British Geological Survey, London. ANDREWS, I. J., LONG, D., RICHARDS,P. C., THOMSON,A. R., BROWN,S., CHESHER, J. A. & McCORMAC. 1990. United Kingdom offshore regional report: the geology of the Moray" Firth. HMSO, London. ARTHURTON, R. S. t980. Rhythmic sedimentary sequences in the Triassic Keuper Marl (Mercia Mudstone Group) of Cheshire, northwest England. Geological Journal, 15, 43-58. BALME, B. E. 1980. Palynology of Permian-Triassic boundary beds at Kap Stosch, East Greenland. Meddelelser om Gronland, 200, (6) 1-37. BARR, K. W., COLTER,V. S. & YOUNG, R. 1981. The geology of the Cardigan BaySt George's Channel Basin. In: ILLING,L. V. & HOBSON,G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 432 443. BENNET, G., COPESTAKE,P. 8r HOOKER,N. P. 1985. Stratigraphy of the Britoil 72/ 10-1A well, Western Approaches. Proceedings of the Geologists' Association, 96, 255-261. BENTON, M. J. 1984. Tooth form, growth, and function in Triassic rhynchosaurs (Reptilia, Diapsida), Palaeontology, 27, 737-776. - - - 1990. The species of Rhynchosaurus, a rhynchosaur (Reptilia, Diapsida) from the Middle Triassic of England. Philosophical Transactions of the Royal Society of London, B328, 213-306. -t~ WALKER, A. O. 1985. Palaeoecology, taphonomy, and dating of P e r m ~ Triassic reptiles from Elgin, north-east Scotland. Palaeontology, 2 8 , 207-234. BERNERS, H-P., BOCK,H., COUREL,L., DEMONFAUCON,A., HARY, A., HENDRIKS,F., MIdLEER, E., MULLER,A., SCHRADER,E. St. WAGNER,J. F. 1984. Vom Westrand des Germanischen Trias-Beckens zum Ostrand des Pariser Lias-Beckens: Aspeckte der Sedimentationsgeschichte. Jahresberichte und Mitteilungen des Oberrheinischen Geologischen Vereines, 66, 357-395. BOURY, J., STEVAUX,J. & WINNOCK,E. 1977. Le Trias d'Aquitaine et seN relations avec le Trias des Pyr~nres et du bassin de l'Ebre. Bulletin du Bureau de Recherche Gbologiques et Mini~res, (2me srr.) IV, no.3, 215-224. BROOKS,J. & GLENNIE,K. W. (eds) 1987. Petroleum Geology of North West Europe. Graham & Trotman, London. BROOKS,J., GOLF, J. C. & VAN HOORN, B. (eds). 1986. Habitat of Palaeozoic Gas in N.W. Europe. Geological Society, London, Special Publication, 23. CHADWICK, R. A. 1985. Seismic reflection investigations into the stratigraphy and structural evolution of the Worcester Basin. Journal of the Geological Society, London, 142, 187-202. --, KENOLTY, N. & WHITTAKER, A. 1983. Crustal structure beneath southern England from deep seismic reflection profiles. Journal of the Geological Society, London, 140, 893-911. - & SMITH, N. J. P. 1988. Evidence of negative structural inversion beneath central England from new seismic reflection data. Journal of the Geological Society, London, 145, 519-522. CHAPMAN,T. J. 1989. The Permian to Cretaceous structural evolution of the Western Approaches Basin (Melville sub-basin), UK. In: COOPER, M. A. & WILLIAMS, G. D. (eds) Inversion Tectonics. Geological Society, London, Special Publication, 44, 177-200. CHEADLE, M. J., MCGEARY, S., WARNER, M. R. & MATTHEWS, D. H. 1987. Extensional structures on the western UK continental shelf: a review of evidence from deep seismic profiling. In: COWARD,M. P., DEWEY,J. F. & HANCOCK,P. L. (eds) Continental Extensional Tectonics. Geological Society, London, Special Publication, 28, 445-465. CLEMMENSEN,U B. 1978. Alternating aeolian, sabkha and shallow-lake deposits from the Middle Triassic Gipsdalen Formation, Scoresby Land, East Greenland. Palaeogeography, Palaeoclimatology, Palaeoecology, 24, 111-135. -1987. Complex star dunes and associated aeolian bedforms. Hopeman Sandstone (Permo-Triassic), Moray Firth Basin, Scotland. In: FROSTICK,L. E. & REID, I. (eds) Desert Sediments: Ancient and Modern. Geological Society, London, Special Publication, 35, 213-231. - - , JACOBSEN,V. W. d~ STEEL, R. J. 1980. Some aspects of Triassic sedimentation and basin development, East Greenland, North Sea, In: The Sedimentation of the North Sea Reservoir Rocks, Norsk Petroleumsforening, Article XVII, Proceedings Geilo Conference. COLIN, J. P., LEHMANN,R. A. St. MORGAN,B. E. 1981. Cretaceous and Late Jurassic biostratigraphy of the North Celtic Sea Basin, offshore Southern Ireland. In: NEALE, J. W. 8~. BRASIER,M. D. (eds) Microfossilsfrom Recent and Fossil Shelf Seas. The British Micropalaeontological Society/Ellis Horwood, Chichester 122-155. COPE, J. C. W., GETTY,T. A., HOWARTH,M. K., MORTON,N. & TORRENS, H. S. 1980. A Correlation of Jurassic Rocks in the British Isles: Part l: Introduction and Lower Jurassic. Geological Society, London, Special Report, 14. DESRAND-PASSARD,S. (co-ordinator). 1980. Synthrse grologique du Bassin de Paris: Volume 2, Atlas. Mkmoires du Bureau de Recherche G~ologiques et Minidres, 102.
DONOVAN,D. T., CURTIS,M. T. & CURTIS,S. A. 1989. A psiloceratid ammonite from the supposed Triassic Penarth Group of Avon, England. Palaeontology, 32, 231-235. FAERSETH,R. B., MACINTYRE,R. M. & NATERSTAD,J. 1976. Mesozoic alkaline dykes in the Sunnhordland region, western Norway: ages, geochemistry and regional significance. Lithos 9, 331-345. FISHER, M. J. 1979. The Triassic palynofloral succession in the.Canadian Arctic Archipelago. AASP Contributions Series, 5B, 83-100~ - - & MUDGE, D. C. 1986. Triassic. In: GLENNIE,K. W. (ed.) Introduction to the Petroleum Geology of the North Sea, 3rd Edition, Blackwell, Oxford. 191-218.
FITCH, F. J., MILLER, J. A. & THOMPSON, D. B. 1966. The palaeogeographic significance of isotopic age determinations on detrital micas from the Triassic of the Stockport-Macclesfield district, Cheshire, England. Palaeogeography, Palaeoclimatology, Palaeoecology, 2, 28 I-312. FORD, T. D. 1969. The stratiform ore deposits of Derbyshire. Proceedings of the 15th Inter-University Geological Congress, 1967, 73-96. FORSTER, S. C. • WARRINGTON,G. 1985. Geochronology of the Carboniferous, Permian and Triassic. In: SNELLING,N. J. (ed.) The Chronology of the Geological Record. Geological Society, London, Memoir, 10, 99-113. FRANCIS, E. H. 1959. The Rhaetic of the Bridgend District, Glamorganshire. Proceedings of the Geologists'Association, 70, 158-178. FRASER, N. C. 1988. Latest Triassic terrestrial vertebrates and their biostratigraphy. Modern Geology, 13, 125-140. - - , WALKDEN,G. M. ~,s STEWART,V. 1985. The first pre-Rhaetic therian mammal. Nature, 314, 161-163. - & UNWlN, D. M. 1990. Pterosaur remains from the Upper Triassic of Britain. Neues Jahrbuch fiir Geologic und Paldontologie, Monatshefte, 1990, H.5, 272282. FROSTICK,L. E. & REID, I. 1987. Tectonic control of desert sediments in rift basins ancient and modern. In: FROSTICK,L. E. & REID, I. (eds) Desert Sediments: Ancient and Modern. Geological Society, London, Special Publication, 35, 5368. HARRISON, R. K. 1971. The petrology of the Upper Triassic rocks in the Llanbedr (Mochras Farm) Borehole. Institute of Geological Sciences, Report 71/18.37-72. - - , JEANS,C. V. 8~. MERRIMAN,R. J. 1979. Mesozoic igneous rocks, hydrothermal mineralisation and volcanogenic sediments in Britain and adjacent regions. Bulletin of the Geological Survey of Great Britain, 70, 57-69. HARRISON, W. J. 1876. On the occurrence of the Rhaetic Beds in Leicestershire. Quarterly Journal of the Geological Society of London, 32, 212-218. HITCHEN, K. & RITCHIE,J. D. 1987. Geological review of the West Shetland area. In: BROOKS,J. & GLENNIE,K. W. (eds) Petroleum Geology of North West Europe, Graham & Trotman, London. 737-749. HOLLOWAY,S., MILODOWSKI,A. E., STRONG,G. E. & WARRINGTON,G. 1989. The Sherwood Sandstone Group (Triassic) of the Wessex Basin, southern England. Proceedings of the Geologists" Association, 100, 383-394. HOLSER, W. T. & WILGUS,C. K. 1981. Bromide profiles of the R6t salt, Triassic of northern Europe, as evidence of its marine origin. Neues Jahrbueh fiir Mineralogie Monatshefte, 1981, H.6, 267-276. HORTON, A., POOLE, E. G., WILLIAMS,B. J., ILLING,V. C. & HOBSON,G. D. 1987. Geology of the country around Chipping Norton. Memoir of the British Geological Survey. ILLING, L. V. & HOBSON, G. D. (eds) 1981. Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London. IVIMEY-COOK,H. C. 1978. In: POOLE, E. G. Stratigraphy of the Withycombe Farm Borehole, near Banbury, Oxfordshire. Bulletin of the Geological Survey of Great Britain, 68, 13. JACKSON, D. I., MULHOLLAND, P., JONES, S. M. 8g. WARRINGTON,G. 1987. The geological framework of the East Irish Sea Basin. In: BROOKS,J. & GLENNIE, K. W. (eds) Petroleum Geology of North West Europe, Graham & Trotman, London. 191-203. JENNER, J. K. 1981. The structure and stratigraphy of the Kish Bank Basin. In: ILLING,L. V. & HOBSON,G. D. (eds) Petroleum Geology of the Continental Shelf of North West Europe. Heyden, London. KAMEREING,P. 1979. The geology and hydrocarbon habitat of the Bristol Channel Basin. Journal of Petroleum Geology, 2, 75-93. KENT,P. E. 1947. A deep boring at North Creake, Norfolk. Geological Magazine, 84, 2-18. KIRTON, S. R. & HITCHEN,K. 1987. Timing and style of crustal extension N of the Scottish mainland. In: COWARD, M. P,, DEWEY, J. F. 8r HANCOCK,P. L. (eds) Continental Extensional Tectonics. Geological Society, London, Special Publication, 28, 501-510. LAKE,S. D. & KARNER,G. D. 1987. The structure and evolution of the Wessex Basin, southern England: an example of inversion tectonics. Tectonophysics, 137, 347378. LERVIK, K. S., SPENCER, A. M. & WARRINGTON, G. 1989. Outline of Triassic stratigraphy and structure in the central northern North Sea. In: COLLINSON, J. D. (ed.) Correlation in Hydrocarbon Exploration, Graham & Trotman, London, 173-189. LINSLEY,P. N., POTTER,H. C., MCNAB, G. & RACHER,D. 1980. The Beatrice Field, Inner Moray Firth, U.K. North Sea. American Association of Petroleum Geologists Memoir, 30, 117-129. LOTT, G. K., SOBEY,R. A., WARRINGTON,G. & WHITTAKER,A. 1982. The Mercia Mudstone Group (Triassic) in the western Wessex Basin. Proceedings of the Ussher Society, 5, 340-346. - & WARRINGTON,G. 1988. A review of the latest Triassic succession in the U.K. sector of the southern North Sea Basin. Proceedings of the Yorkshire Geological Society, 47, 139-147. MANSPEIZER,W. (ed.) 1988. Triassic-Jurassic Rifting: Continental Breakup and the Origin of the Atlantic Ocean and Passive Margins. Developments in Geotectonics, 22. Elsevier, Amsterdam. MARSHALL, J. E. A. & WHITESIDE, D. I. 1980. Marine influence in the Triassic 'uplands'. Nature, 287, 627-628. MAYALL, M. J. 1981. The Late Triassic Blue Anchor Formation and the initial Rhaetian marine transgression in south-west Britain. Geological Magazine, 118, 377-384. - - - 1983. An earthquake origin for synsedimentary deformation in a late Triassic (Rhaetian) lagoonal sequence, southwest Britain. Geological Magazine, 120, 613-622. MAP Tr4b. Extent of the Mercia Mudstone Group, including the Arden Sandstone Member and its correlatives (Association C), at outcrop and subcrop in parts of southern and central England. Present outcrops of pre-Mercia Mudstone Group rocks (from 1:63 360 geological map, southern sheet, British Geological Survey) and concealed areas lacking Mercia Mudstone Group deposits (from Whittaker et al. 1985a) (brown). Outcrop of Association C beds (red) (from British Geological Survey I :50 000 geological sheets and approved 1: I0 000 maps (with the approval of the Director, BGS), Elliott (1961), Ruffell & Warrington (1988); approximate concealed limit on ticked side of coarse pecked line. 1 to 6: named representatives of Association C, with biota. General palaeocurrent directions (from cross bedding) indicated by black sectors in circles, and range of ripple crest orientation (in 3a) by stippled sectors (2 and 3 from Warrington 1968; 5 from Warrington & Williams 1984, Ruffell & Warrington 1988).
105
106
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MI~GNIEN, C. & Mf~GNIEN, F. (eds) 1980. Synthrse grologique du Bassin de Paris: Volume l, Stratigraphie et Palrogrographie. M~moires du Bureau de Recherche Gbologiques et Mini&es, 101. MILNER, A. R., GARDINER,B. G., FRASER,N. C. & TAYLOR, M. A. 1990. Vertebrates from the Middle Triassic Otter Sandstone Formation of Devon. Palaeontology, 33, 873-893. NAYLOR, D. 8s SHANNON, P. 1982. Geology of Offshore Ireland and West Britain. Graham & Trotman, London. PARRISH, J. T. & CURTIS, R. L. 1982. Atmospheric circulation, upwelling, and organic-rich rocks in the Mesozoic and Cenozoic eras. Palaeogeography, Palaeoclimatology, Palaeoecology, 40, 31-66. - - , ZIEGLER,A. M. & SCOTESE,C. R. 1982. Rainfall patterns and the distribution of coals and evaporites in the Mesozoic and Cenozoic. Palaeogeography, PaiReDclimatology, Palaeoecology, 40, 67-101. PENN, I. E. 1987. Geophysical logs in the stratigraphy of Wales and adjacent offshore and onshore areas. Proceedings of the Geologists' Association, 98, 275-314. --, CHADWICK, R. A., HOLLOWAY, S., ROBERTS, G., PHARAOH, T. C., ALLSOP, J. M., HUEBERT, A. G. & BURNS, I. M. 1987. Principal features of the hydrocarbon prospectivity of the Wessex-Channel Basin, UK. In: BROOKS, J. & GEENNIE, K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 109-118. PIASECKI, S. 1984. Preliminary palynostratigraphy of the Permian-Lower Triassic sediments in Jameson Land and Scoresby Land, East Greenland. Bulletin of the Geological Society of Denmark, 32, 139-144. POLLARD,J. E. 1981. A comparison between the Triassic trace fossils of Cheshire and south Germany. Palaeontology, 24, 555-588. RIDDLER, G. P. 1981. The distribution of the Rot Halite Member in North Yorkshire, Cleveland and North Humberside. Proceedings of the Yorkshire Geological Society, 43, 341-346. RITCHIE, J. D., SWALLOW,J. L., MITCHELL, J. G. & MORTON, A. C. 1988. Jurassic ages from intrusives and extrusives within the Forties igneous province. Scottish Journal of Geology, 24, 81-88. ROBESON, D., BURNETT, R. O. & CLAYTON,G. 1988. The Upper Palaeozoic geology of the Porcupine, Erris and Donegal basins, offshore Ireland. Irish Journal of Earth Sciences, 9, 153-175. ROBINSON, K. W., SHANNON,P. M. & YOUNG, D. G. G. 1981. The Fastnet Basin: an integrated analysis. In: ILLING, L. V. & HOBSON, G. D. (eds). Petroleum Geology of the Continental Shelf of North- West Europe. Heyden, London, 444 454. ROBINSON, P. L. 1973. Palaeoclimatology and Continental Drift. In: TARLING,D. H. & RUNCORN,S. K. (eds). Implications of Continental Drift to the Earth Sciences, 1,451-476. Academic, London. RoE, S-L. & STEEL,R. 1985. Sedimentation, sea-level rise and tectonics at the TriassicJurassic boundary (Statfjord Formation), Tampen Spur, northern North Sea. Journal of Petroleum Geology, 8, 163-186. RUEEELL, A. & WARRINGTON, G. 1988. An arenaceous member in the Mercia Mudstone Group (Triassic) west of Taunton, Somerset. Proceedings of the Ussher Society, 7, 102-103. SIMMS, M. J. & RUFEELL,A. H. 1990. Climatic and biotic change in the late Triassic. Journal of the Geological Society, London, 147, 321-327. SMITH, D. B. 1989. The late Permian palaeogeography of north-east England. Proceedings of the Yorkshire Geological Society, 47, 285-312. SMITH, S. A. 1990. The Sedimentology and accretionary styles of an ancient gravelbed stream: the Budleigh Salterton Pebble Beds (Lower Triassic), southwest England. Sedimentary Geology, 67, 199-219. STEEL~ R. J. & THOMPSON, D. B. 1983. Structures and textures in Triassic braided stream conglomerates ('Bunter' Pebble Beds) in the Sherwood Sandstone Group, North Staffordshire, England. Sedimentology, 30, 341-367. - & RYSETH, A. 1990. The Triassic-early Jurassic succession in the northern North Sea: megasequence stratigraphy and intra-Triassic tectonics. In: HARDMAN, R. F. P. & BROOKS, J. (eds) Tectonic" Events Responsible for Britain's Oil and Gas Reserves. Geological Society, London, Special Publication, 55, 139-168. ST~VAUX, J. & W1NNOCK, 1~. 1974. Les bassins du Trias et du Lias infrrieur d'Aquitaine et leurs 6pisodes 6vaporitiques. Bulletin de la Sociktk g~ologique de France, XVI, 679~595. SWlFT, A. 1989. First records of conodonts from the late Triassic of Britain. Palaeontology, 32, 325-333. TA]'E, M. P. & DOI~SON, M. R. 1989. Late Permian to early Mesozoic rifting and sedimentation offshore NW Ireland. Marine and Petroleum Geology, 6, 49-59. TAYLOR, S. R. 1983. A stable isotope study of the Mercia Mudstones (Keuper Marl) and associated sulphate horizons in the English Midlands. Sedimentology, 30, 11-31. TOLLMANN, A. & KRISTAN-TOLLMANN,E. 1985. Paleogeography of the European Tethys from Paleozoic to Mesozoic and the Triassic relations of the eastern part of Tethys and Panthalassa. In: NAKAZAWA, K. & DICKINS, J. M. (eds) The
Tethys: Her Paleogeography and Paleobiogeography from Paleozoic to Mesozoic, 3-22. Tokai University Press. TOTER, E. T. 1984. The Trias and its ammonoids: the evolution of a time scale. Geological Survey of Canada Miscellaneous Report, 35.
1988. Definition of the Permian-Triassic (P-T) boundary: the question of the age of the Otoceras Beds. Memorie della Societd Geologica Italiana, 34, 291-301. TUCKER, M. E. 1978. Triassic lacustrine sediments from South Wales: shore-zone clastics, evaporites and carbonates. In: MATTER, A. & TUCKER, M. E. (eds). Modern and Ancient Lake Sediments, Special Publication of the International Association of Sedimentologists, 2, 205-224. TUCKER, R. M. & ARTER, G. 1987. The tectonic evolution of the North Celtic Sea and Cardigan Bay basins with special reference to basin inversion. Teetonophysics, 137, 291-307. VAN HOORN, B. 1987. The South Celtic Sea/Bristol Channel Basin: origin, deformation and inversion history. Teetonophysies, 137, 309-334. VOLLSET, J. & DORL A. G. 1984. A revised Triassic and Jurassic lithostratigraphic nomenclature for the Norwegian North Sea. Norwegian Petroleum Directorate - -
Bulletin, 3. WARRINGTON, G. 1968. The stratigraphy and palaeontology of the "Keuper" Series (upper Triassic,) in central England. PhD thesis, University of London. -1980. The Alderley Edge mining district. The Amateur Geologist, 8, 4-13. --
1981. The indigenous micropaleontology of British Triassic shelf sea deposits.
In: NEALE, J. W. & BRASIER, M. D. (eds) Microfossils from Recent and Fossil Shelf Seas, 61-70. The British Micropalaeontological Society/Ellis Horwood, Chichester. 1983. Late Triassic and earliest Jurassic palynomorph assemblages from the Western English Channel and neighbouring areas. Proceedings of the Ussher Society, 5, 473~,76. -1986. British Triassic palaeontology: supplement 10. Proceedings of the Ussher Society, 6, 420-421. - 1990. The Permian of Devon, England. Review of Palaeobotany and Palynology, 66, 263-272. - - , AUDLEY-CHARLES,M. G., ELL1OTT, R. E., EVANS, W. B., IVIMEY-COOK,H. C., KENT, P . E . , R O B I N S O N , P. L., Sno-rroN, F. W. & TAYLOR, F. M. 1980. A Correlation of Triassic Rocks in the British Isles. Geological Society, London, Special Report, 13. - & IVIMEY-COOK, H. C. 1990. Biostratigraphy of the late Triassic and early Jurassic: a review of type sections in southern Britain. Cahier Universit~ Catholique, Lyon, srr. Science, no. 3, 207-213. - & WHITTAKER, A. 1984. The Blue Anchor Formation (late Triassic) in Somerset. Proceedings of the Ussher Society, 6, 100-107. - & WILLIAMS,B. J. 1984. The North Curry Sandstone Member (late Triassic) near Taunton, Somerset. Proceedings of the Ussher Society, 6, 82-87. WATERS, R. A. & LAWRENCE, D. J. D. 1987. Geology of the South Wales Coalfield, Part III, the country around Cardiff. Memoir of the British Geological Survey. WHITTAKER, A. (ed.). 1985. Atlas of Onshore Sedimentary Basins in England and Wales." Post-Carboniferous Tectonics and Stratigraphy. Blackie, Glasgow & London. -& GREEN, G. W. 1983. Geology of the country around Weston-super-Mare. Memoir of the Geological Survey of Great Britain. , HOLLIDAY,D. W. & PENN, I. E. 1985. Geophysical Logs in British Stratigraphy. Geological Society, London, Special Report, 15. WIEDMANN, J., FABRICIUS,F., KRYSTYN, L., REITNER, J. & URLICHS, M. 1979. 1Dber Umfang und Stellung des Rhaet. Newsletters on Stratigraphy, 8, ! 33-152. WILEE, W. 1970. Plaesiodictyon mosellanum n.g., n.sp., eine mehrzellige Griinalge aus dem Unteren Keuper von Luxemburg. Neues Jahrbuch far Geologie und Paliiontologie, Monatshefte, 1970. H.5, 283-310. -1947. A monograph of British Triassic scorpions. Palaeontographical Society, London. -1950. The Palaeogeography of the Midlands. University Press, Liverpool. WILSON, A. A. 1990. The Mercia Mudstone Group (Trias) of the East Irish Sea Basin. Proceedings of the Yorkshire Geological Society, 48, 1-22. WOODHALL, D. & KNOX, R. W. O'B. 1979. Mesozoic volcanism in the northern North Sea and adjacent areas. Bulletin of the Geological Survey of Great Britain, 70, 34-56. WURSTER, P. 1964. Geologie des Schilfsandsteins. Mitteilungen aus dem Geologischen Staatsinstitut in Hamburg, 33. YOUNG, B. & LAKE, R. D. 1988. Geology of the country around Brighton and Worthing. Memoir of the British Geological Survey. ZAKHAROV, Y. D. 1987. The problem of Stage-subdivision of the Lower Triassic. Albertiana, 6, 14-21. ZAPFE. H. 1983. DaN Forschungsprojekt "Triassic of the Tethys Realm" (IGCP Proj.4) Abschlussbericht. In: ZAPFE, H.(ed.) Neue Beitr/ige zur Biostratigraphie der Tethys-Trias. Osterreichische Akademie der Wissenschaften, Schriftenreihe der Erdwissenschaftlichen Kommissionen Band 5, 7-16. ZIEGLER, P. A. 1982. Geological Atlas of Western and Central Europe. Shell Internationale Petroleum Maatschappij B. V., Den Haag. - (ed.) 1987a. Compressional intra-plate deformations in the Alpine Foreland. Tectonophysics, 137. - 1987b. Evolution of the Western Approaches Trough. Tectonophysics, 137, 341-346. ---
J u rassic M. J. B R A D S H A W ,
J. C. W . C O P E ,
D. W . C R I P P S , D. T. D O N O V A N , I. M . W E S T & W . A. W I M B L E D O N
The interplay of regional or global sea-level changes and continuing crustal extension related to rifting in the North Atlantic and Tethyan megarifts dominated the palaeogeographical evolution of the area. The sea-level rise that commenced in the Rhaetian continued into the Jurassic to usher in a new phase of predominantly marine sedimentation across northwest Europe. The climate was warm and humid, the seas generally shallow. Land areas became well vegetated, as attested by the abundance of fossil driftwood in marine as well as non-marine sediments. The region probably lay about 100 south of present latitudes, in an area of overlap of Tethyan and Boreal marine realms. For much of the Jurassic, sea levels continued to rise but two major falls occurred, coinciding with accelerated extensional movements in the area. The first was during the mid-Jurassic ('mid-Cimmerian phase') when there was a major regional upwarp centred on the North Sea, and the second across the Jurassic/Cretaceous boundary ('late Cimmerian phase') when a global fall in sea level coincided with an important phase of rifting and block faulting. Major structural trends generally follow previously established lines. The basic structural framework for the Jurassic maps has been compiled from numerous sources, including Andrews & Brown (1987); Dunning (1985); Evans et al. (1982); Gardiner & Sheridan (198 i); Van Hoorn (1987); Thomas et al. (1985); Whittaker (1985); Ziegler (1982, 1987); and papers in Brooks & Glennie (1987). Fault trends are generalized and selected to pick out the main structural features controlling the geography; their occurrence on any individual map does not mean that they were necessary moving actively at that time but does indicate that they are believed to have played at least a passive role. in controlling the contemporary geography. For present-day onshore areas biostratigraphical control is generally excellent, based on ammonite biozonation of unsurpassed quality (see Cope et al. 1980a,b). Dating of offshore sequences is based primarily on microfossils and palynomorphs and is inevitably more generalized, so that the inclusion of offshore facies on any particular 'snapshot' map is subjective but based on a realistic interpretation of the available evidence. PFR EARLY JURASSIC
Introduction Relative sea levels rose theough most of Early Jurassic time, and by maximum transgression much of northwest Europe was probably submerged. However, within the map area accurate geographic reconstruction is hampered by the patchy preservation of sediments. Mudrocks form the dominant lithology though sandstones and ironstones accumulated at times. The main lithostratigraphic divisions are reviewed in Cope et al. (1980a). Few sediments are preserved in the northern part of the area. Here, midJurassic uplift in the central North Sea resulted in extensive erosion of Lower Jurassic sediments, though remnants are preserved in the Fiskebank Sub-basin and Norwegian Central Graben. Away from the uplift area there is a thick sequence in the Inner Moray Firth Basin (though with a mid Pliensbachian to mid Toarcian break), the North Viking Graben and the Danish Sub-basin. South of the Mid North Sea High Lower Jurassic sediments are more widely preserved. In the southern North Sea region there are three main areas, separated by areas where sediments were eroded away during the late Cimmerian movements at the end of the Jurassic. The largest extends eastwards from outcrop in eastern England to form an extensive subcrop offshore. It reaches a maximum thickness of about 900 m in the Sole Pit Trough (Deegan & Scull 1977; Brown 1986). A second area occurs in the Danish and nearby Dutch sectors of the Central Graben. In the Danish Central Graben only Hettangian-Sinemurian sediments (Fjerritslev Formation) are preserved (Jensen et al. 1986) but in the Dutch sector all stages are represented in the Altena Group. A third area lies further south, in the Broad Fourteens, West and Central Netherlands Basins; the sediments are again assigned to the Altena Group (Rhaetian-Oxfordian). From eastern England sediments crop out round the western margin of the Anglo-Brabant high, to the southwest of which lie two depositional troughs trending northwest to southeast and separated by the Radstock Shelf. Both appear to have undergone the greatest downwarping at their northwestern ends. The northeastern one is probably continuous with the Severn Basin. The southwestern trough passes into the Bristol Channel, where thick Hettangian to early Sinemurian sequences are found on both 9 1992The GeologicalSociety
M . K. H O W A R T H ,
P. F.
RAWSON,
shores, and possibly to the South Celtic Sea Basin. The troughs were probably fault-controlled. In the Wessex Basin Lower Jurassic rocks are best known at outcrop in Dorset. To the east and southeast they have been proved in scattered boreholes in the Weald and Channel Basins, though in ~either basin is the Lias well known. Further east there is an extensive development in the Paris Basin (M+gnien & M+gnien 1980). In the Western Approaches Trough released wells indicate only a few small erosional remnants of (earliest) Liassic sediment beneath the Cretaceous unconformity, except near the French coast where thick sequences are preserved in the Brittany and South Channel basins. On the opposite side of the Cornubian Platform the Lias is preserved extensively in the Fastnet, Celtic Sea and Cardigan Bay basins. Its occurrence over the Irish Sea area is open to speculation, but the Kish Bank Basin contains some 2000 m of Liassic sediments, while in the Keys Basin a seismic unit about 500m thick is possibly the same age (Dobson & Whittington 1979; Jackson et al. 1987). There is also an extensive area of Hettangian to Pliensbachian sediments in Antrim, Northern Ireland, and relict patches occur in the Prees and Carlisle outliers. Lower Jurassic sediments crop out along the west coast of Scotland and have been proved in most of the offshore basins as far north as the West Orkney basin complex (Evans et al. 1982). Further north sediments have yet to be proved in the West Shetland area, but some wells have yielded reworked lower Jurassic marine microfossils of Pliensbachian to Toarcian age (Hitchen & Ritchie 1987). Sediments may occur at depth in the Rockall and Faroe rifts (Hitchen & Ritchie 1987). Mesozoic sediments that were deposited in the East Greenland Rift are still present along the continental margin of southeast Greenland-and the northwest margin of the Hatton Bank, though their age in the latter area is, as yet, undetermined (Bott 1978). Away from the areas of preserved sediment it is often difficult to infer land. Much of Scotland is shown as emergent, and some of the sediments along the western coast do show evidence of land to the east. The East Shetland Platform shed sediment to the east, possibly as a result of subaerial erosion, while the Unst Basin may have been a region of non-marine sedimentation (Johns & Andrews 1985). Whether the 'Scottish Landmass' extended southward to join a 'Pennine High' is dubious. Most authors, at least from Arkell (1933) onwards, have shown the Pennines as a Jurassic land mass. However, the present relief dates only from the Neogene (Walsh et al. 1972), and in the early Jurassic they were probably emergent only in the late Pliensbachian (Map J2b). Arguably the one well established area of non-deposition was the western part of the Anglo-Brabant massif, the 'London Platform'. Gradual transgression of the Lower Lias over the flanks of this positive area is well proven by boreholes (Donovan et al. 1979). However, there is no satisfactory proof that the massif was an island rather than a submerged shoal, though the former has generally been assumed. If so it is likely to have had low relief since there is little change in the character of sedimentation towards it and little or no evidence that it was a source of sediments except possibly in Belgium. The Palaeozoic outcrops of Wales are flanked by thick Lower Jurassic on all sides: at Bridgend in Glamorgan; in the Llanbedr (Mochras Farm) Borehole; and, at slightly greater distances, in the Prees outlier and wells in the Cardigan Bay and Celtic Sea basins. These occurrences owe their proximity to the Palaeozoics to post-Jurassic tectonics. There is no reason to believe that the Welsh Palaeozoics formed an upland area in the early Jurassic (see also Cope 1984). Similar considerations apply to Cornubia. Thick Hettangian and Sinemurian rocks occur faulted close to Upper Palaeozoics near Watcher in Somerset, and offshore only a kilometre or two from the steep North Devon coast (Lloyd et al. 1973). Here there is slightly more cause for doubt because the late Palaeozoic granites may have formed a buoyant area of the crust over a long period, and may have formed a shoal or an island. If so, it had little effect on the nature of the surrounding sediments. The occurrence and extent of an Irish landmass is uncertain. Reconstruction of the northwest sector of the area is equally conjectural but essentially follows that of Ziegler (1982, enclosure 18). There is structural evidence for non-deposition of Lower Jurassic sediments on the Hebrides Platform (Kent 1975). Vulcanicity occurred in two widely separated areas. Extrusion of Hettangian basalts in the outer part of the Western Approaches Trough (see Map J l) was related to subsidence in the nearby Biscay Rift, while local tuffaceous bands in the Lower Jurassic of the central North Sea were precursors 107
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of extensive Mid-Jurassic vulcanicity there (Deegan & Scull 1977). Dykes were intruded into the Egersund Sub-Basin during the Early Jurassic (Furnes et al. 1982). DTD, MKH
borehole 77/8 in the North Lewis Basin has early Lower Jurassic marine mudstones. DTD
J2a: Early Pliensbachiaa (Carixian) Jl: Early Hettangian The major northward transgression initiated during the Rhaetian continued in Hettangian times. Map Jl represents its possible extent in the planorbis Zone. Non-marine sediments occur to the east of the Shetland Platform, where the arenaceous Statfjord Formation is well developed in the North Viking Graben and apparently extends into the South Viking Graben (Harris & Fowler 1987, fig. 2). The general distribution of these sediments is based on Skarpnes et al.'s (1980, fig. 4) map. To the southeast, there are scattered relics of the non-marine Gassum Formation in the Fiskebank Sub-basin (Hamar et aL 1983), and it was not until the latest Sinemurian that marine transgression reached there from the Danish Basin. In the inner Moray Firth marine sedimentation started earlier; here the Dunrobin Bay Formation (Hettangian-mid Pliensbachian) consists of shales with minor sandstones, which record an upward transition from non-marine to open marine conditions by the Pliensbachian (Brown 1986; Andrews & Brown 1987). In the southern half of the map area Hettangian marine sediments are known from several basins. They are sometimes argillaceous but frequently occur as thin, rhythmic alternations of limestone and shale, which have received local names such as 'Blue Lias' or 'Hydraulic Limestones'. The limestones are largely secondary but they probably reflect a primary sedimentary rhythm, the individual limestone/shale couplets (c. 1 m thick) possibly reflecting astronomically induced climatic cycles (House 1985). In eastern England the Hydraulic Limestones occur on the East Midlands Shelf and the more argillaceous 'calcareous shales' in the Cleveland Basin. The shale/limestone facies apparently extends offshore where limestone beds are generally more numerous in the lowest Lias (Rhys 1974); they are common in the lowest 15 m in the type well for the basal Jurassic, BP 48/6-5. By analogy with the Cleveland Basin, the facies further into the Sole Pit Basin is indicated as less calcareous though there is no firm evidence for this subtle distinction. The approximate extent of sediments is modified from the generalized Lias subcrop shown by Fyfe et al. (1981, fig. 2). In the southern part of the Danish Central Graben a small patch of marine shales belongs to the Fjerritslev Formation (Hettangian-Sinemurian); their distribution is based on Jensen et al. (1986). The nearby Dutch Central Graben preserves a larger area of mudrocks similar to the Fjerritslev Formation but here assigned to the Aalburg Shale Formation (NAM 1980). Similar mudrocks, placed in the same formation, occur in the Broad Fourteens, Central and West Netherlands Basins; their approximate distribution is based on the occurrence of the Altena Group (Van Wijhe 1987, fig. 2). In the Channel Basin the lowest Lias in well 98/22-2 consists of massive limestones, which pass up into more typical Blue Lias facies. A comparable sequence occurs at the eastern end of the Western Approaches Trough, where slightly argillaceous limestones predominate in the lowest Hettangian of well 88/2-1 (Evans et al. 1981). Further west, released wells indicate that Hettangian sediments are missing along the axial region of the Trough, except at the western extremity where 72/10-1A and 73/1-1A proved mudrocks, the latter with thick limestone interbeds. Within the Hettangian sequence of 71/10-1A are 33 m of weathered basaits and tufts with thin claystone interbeds. A thick sequence is preserved in the Brittany Basin, where well LIN 1 penetrated a Hettangian to Sinemurian carbonate formation, while in BEL 1 the lowest Hettangian is represented by black, slightly calcareous silty clays. In the areas flanking the Bristol Channel, small outcrops of Upper Palaeozoic rocks, which formed an archipelago off the western end of the Anglo-Brabant massif, gave rise to local developments of coarse bioclastic, and sometimes conglomeratic facies (Sutton Stone, Brockley Down Limestone). In one or two cases actual shorelines may be preserved. It is doubtful whether any of these shoals or islands continued to affect sedimentation after the beginning of the Sinemurian. In the Celtic Sea area the Croyde Formation is of Hettangian to Sinemurian age. The formation is predominantly cyclic (Blue Lias facies) but southeastwards, towards the Fastnet Basin, the lowest part passes laterally into limestones of the Woolacombe Member (Millson 1987 and released wells). The Lower Hettangian is probably represented also in the Liassic sequence of the Kish Bank Basin and Keys Basin. In the Prees and Carlisle outliers, Antrim and Mull the planorbis Zone is preserved in typical Blue Lias facies. Records of this zone in a Tertiaryvent on Arran are based on a misidentified ammonite (Donovan in Cope et al. 1980a). The northward extent of early Hettangian seas is speculative and the extent of non-marine sedimentation may be underestimated on the map; it is possible that the North Minch and West Orkney basin complex and Faeroe Rift were non-marine, Certainly the oldest marine Jurassic sediments in Skye and Raasay are probably late Hettangian, while in the West Orkney basin complex some non-marine, redbed occurrences could be as young as earliest Jurassic, particularly the red siltstones of BGS borehole 72/34 (Evans et al. 1982). The oldest reworked Lower Jurassic (marine) microfossils recorded from the West Shetland area are Pliensbachian (Hitchen & Ritchie 1987). On the other hand the onset of marine cOnditions must have been quite rapid as
The map corresponds, as far as possible, to the time represented by the ibex Zone. It is intended to show the maximum extent of the Lower Liassic transgression, when the structural highs were at least partially flooded. However, the extent of land remaining is still largely conjectural. The early Hettangian non-marine area to the east of the Shetland Platform was transgressed during the later Hettangian and Sinemurian, and by Pliensbachian times marine shales of the Dunlin Group were widespread. The facies distribution is based primarily on Skarpnes et al. (1980, fig. 5). Biostratigraphic resolution is not fine enough to show whether the lowest part of the more silty and sandy Cook Member may be as old as the ibex Zone. Skarpnes et al. (1980) showed land to the south, separating this region from the Fiskebank sub-basin, but the Dunlin Group certainly extends into the South Viking Graben (e.g. Harris & Fowler 1987, fig. 2) and the adjacent part of the Vestland Arch probably lay beneath shallow water. The Fiskebank Sub-basin itself had been transgressed from the east by the beginning of the Pliensbachian, when deposition of the Fjerritslev Formation shales commenced; their generalized distribution is from Hamar et al. (1983, fig. 3a). Shales with marginal sands also occur in the inner Moray Firth Basin (Brown 1986) where the highest part of the Dunrobin Bay Formation is o f early Pliensbachian age. To the south of the Mid North Sea High and Scottish Landmass Lower Pliensbachian sediments are more widely distributed. Here, the rhythmic sedimentation initiated in the Hettangian probably continued through much of the Lower Lias (Sellwood 1970) but after early Sinemurian times there was insufficient carbonate to give rise to well developed limestones. However, a rhythmic pattern of about 1 m is seen on many geophysical logs through mudstone lithology. The later Sinemurian and earliest Pliensbachian rocks are chiefly mudstones, but in many sections silt, and more rarely fine sand, begins to be noted as one passes up from thejamesoni into the ibex Zone. It is not considered useful to distinguish between different types of argillaceous rocks, e.g. shales and mudstones, because their distinctive features are largely developed at outcrop after weathering and are less apparent in the borehole records on which the present compilation largely depends. It seems likely, however, that the laminated dark bituminous shales (paper shales on weathering) which are fairly common at lower horizons (Hallam 1960) are not much in evidence in the ibex Zone. The monotonous facies of the later Sinemurian and Pliensbachian is varied locally by the accumulation of ferruginous sediments. The two principal occurrences are the Frodingham Ironstone (Lincolnshire) in mid Sinemurian times, and the Jamesoni Limestone (Avon and Somerset) somewhat later, at the beginning of the Pliensbachian. Both these deposits correspond to areas of locally condensed deposition and presumably indicate positive movement, or at least relative stability, of the sea floor. Onshore occurrences in the southern area are summarized by Howarth (in Cope et al. 1980a). A broad belt of shales and silty shales extends from eastern England and the adjacent offshore area to the Bristol Channel and Dorset. Scattered boreholes have proved similar facies in the Weald and Channel Basins. They are generally missing in the Western Approaches Trough but have been proved in one well (8/2-1) in the eastern part of the Trough. The main facies change is in the northeastern part of the Paris Basin where limestones occur (Mrgnien & Mrgnien 1980). In the Fastnet, Celtic Sea and Cardigan Bay basins cyclic sediments of the Croyde Formation gave way to monotonous mudrocks forming the Kilkhampton Formation (latest Sinemurian and Pliensbachian) MiUson (1987). Similar facies occur in the Prees outlier. The Port More borehole in Antrim proved mudrocks as young as the lower ibex Zone. Marine elastics are also known in the Slyne Trough. On Mull, Raasay and Skye the Pabba Shales occur; in the last two islands these beds pass up into the more arenaceous Scalpa Sandstone, the change occurring within the ibex Zone. Further north several shallow boreholes have proved Lower Jurassic sediments (Evans et al. 1982), but most are poorly dated. The silty mudstone of BGS borehole 77/8 in particular is a probable lithology for the late Carixian and is thus indicated on the map. DTD
J2b: Late Pliensbachian (Domerian) The clay-shale facies that was so widespread in the middle of the Lower Pliensbachian (ibex Zone) was superseded by slowly increasing arenaceous sedimentation later in the Pliensbachian, particularly in the present British onshore area. The earliest occurrence of sands and sandstones is in the upper part of the ibex Zone in Skye and Raasay; sand and silt appeared in Yorkshire and in Gloucestershire in the davoei Zone, and in Dorset, Somerset, and Oxfordshire to Leicestershire in the margaritatus Zone. By the middle of the margaritatus Zone considerable thicknesses of sandstones and sandy shales were being deposited almost everywhere in Britain, with the exception of Lincolnshire (from Grantham to the Humber) which remained argillaceous throughout. This was probably the time of maximum regression of the sea and the greatest extent of land areas, with considerable erosion to produce
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the arenaceous sediments. Deposition of sand reached its peak in western Scotland in the succeeding spinatum Zone, when the thickest and most massive parts of the Scalpa Sandstone were laid down. Over England, however, the deposition of sand ceased suddenly; first in Yorkshire in the middle of the margaritatus Zone when shales with beds and concretions of argillaceous ironstone appeared, then over the rest of England at the base of the spinatum Zone when deposition commenced of the widespread Marlstone Rock Bed. The latter is an oolitic limestone which is sufficiently rich in iron minerals in some areas to be worked as an iron-ore. Map J2b represents the spinatum Zone. The palaeogeography was probably no different from that of the margaritatus Zone because there is no evidence of transgression of the sea, and the same land areas probably 9remained, though of lower relief than in the margaritatus Zone. In fact, the spinatum Zone might have been the time of maximum regression, because the ammonite and brachiopod faunas were divided into small-scale provinces in Britain. The main ammonite genus was Pleuroceras, which evolved into slender-whorled species in the Yorkshire Province north of the Market Weighton High, while more massive-whorled tuberculate species developed in the Southwestern Province in Gloucestershire to Dorset, southwest of the Vale of Moreton shallows. The intervening Midland Province had a very impoverished Pleuroceras fauna, and it is probable that the ironstone facies there acted as a barrier to migration of ammonites between the Yorkshire and Southwestern Provinces. The ammonites of the Hebridean Province were similar to, though less diverse than, those of the Southwestern Province (Howarth 1958, p. xxxvi). Brachiopods were divided into almost identical provinces in the spinatum Zone (Ager 1956). Evidence for the exact position of the land areas shown on Map J2b is not strong. Some account has been taken of the necessity to provide sufficiently large source areas for the considerable amount of sand that was deposited during the margaritatus Zone in Yorkshire and southwest England. Directions of derivation of these sands have not been determined, but it seems unlikely that the Anglo-Brabant landmass was their only source. The islands over and to the west of Cornwall and over South Wales are more probable sources for sediment in southwest England. Preserved sediments put a limit on the extent of a possible South Wales island, but land there could have been a sediment source. In Ireland the evidence is weaker, and a possible landmass is indicated merely to provide the sediments that occur to the south and in the Slyne Trough. Sedimentation in the English Channel extended southwards to virtually the northern tip of the Cotentin Peninsula. The existence of a large island covering most of Brittany and Normandy is well established, and on the eastern side of the Cotentin Peninsula Pliensbachian limestones and basal conglomerates of a near-shoreline facies overlie Lower Palaeozoic rocks. All palaeogeographical reconstructions show a large area of land over much of mainland Scotland in the Lower Jurassic. For the Upper Pliensbachian this is extended down into northern England, to provide the sediments of the Cleveland Basin. The Scalpa sandstones of the Hebrides-Minch basin are readily derived from this landmass; they are exposed on Skye and Mull. The evidence for the extension of the land eastwards into the North Sea comes from boreholes in and around the Viking Graben. The best evidence of a shoreline is northeast of the Shetlands where the sediments in the graben coarsen westwards (Brooks & Chesher 1975, pp 2-7). The distribution of facies is modified from Skarpnes et al. (1980, fig. 6). No Upper Piiensbachian sediments are preserved in the Inner Moray Firth Basin. They are also absent over most of the central North Sea, but mudrocks of this age form part of the Fjerritslev Formation of the Fiskebank Sub-basin (Hamar et al. 1983). To the south of the mid-North Sea High a more extensive area of sediments extends from the outcrop areas of eastern England to the Sole Pit Trough. Howard (1985, p272) noted that the Lias was rarely cored offshore, virtually all the information coming from wireline logs. But in type well 47/15-1X a calcareous sandstone with chamosite ooliths represents the Marlstone Rock Bed (Rhys 1974, p6). This unit produces a distinctive signature on wireline logs in the southern North Sea where it was probably continuous. To the south of the Anglo-Brabant landmass, limestones are extensively developed in the western part of the Paris Basin, passing into mudrocks in the basin centre. Some marginal sands occur close to the Armorican island. Facies distributions are based on M6gnien & M6gnien (1980). Ciaystones with thin limestone bands occur in the Central Channel Basin; they were proved in wells 98/22-1 and 98/22-2 and are probably more widespread. In much of the Western Approaches Trough no sediments are known, but they are preserved in the Brittany Trough (just off the map) and possibly in the South Channel Basin (well LET 1). In the Celtic Sea basins Upper Pliensbachian mudrocks form the upper part of the Kilkhampton Formation (Millson 1987). Over part of the area the highest, more siliciclastic, beds are separated as the Spaunton Member (Miltson 1987), but there is insufficient published evidence to show on the map anything other than a broad area of mudrocks. MKH
J3: Mid Toarcian Although overall the Toarcian was an interval of significant sea level rise, the initial rise (if any) was small. The main basal Toarcian facies change took place in the Cleveland Basin, where the late Pliensbachian shale-ironstone facies gave way to grey shales. During this time (tenuicostatum Zone),
deposition of the Madstone Rock Bed in other areas of England, and of the Scalpa Sandstone in Scotland, continued unaltered. However, there was a significant faunal change, an influx of dactylioceratid ammonites completely replacing the amaltheid faunas. The major change in sea level occurred at the beginning of.thefalciferum Zone, when sedimentation rates increased and mudrocks became widespread. This pattern continued through the succeeding bifrons Zone so that in most parts of Britain both zones are represented by shales with early diagenetic nodules or beds of limestone. A much condensed sequence of limestones was formed in south Dorset, and in western Scotland an iron-rich limestone, the Raasay Ironstone, was deposited during thefalciferum Zone. Major deposition of sand started again in the upper half of the bifrons Zone in the Cotswolds and in higher zones elsewhere in southwest England. So the time of maximum transgression in the Toarcian was probably the lower part of the bifrons Zone, and this is the date of Map J3. The land areas on Map J3 are similar to those on the Late Pliensbachian map, because it is difficult to find evidence for any substantial changes. The Shetland Platform was probably still largely emergent, with mudrocks and marginal sands of the Dunlin Group accumulating .to the east; the distribution of facies is modified from Skarpnes et al. (1980, fig. 7). Sediments may extend further south than shown on the map but are poorly known (Brown 1986, p. 150). They are missing in the Moray Firth Basin, though in the inner basin sedimentation started again later in the Toarcian. Shales of Toarcian age form part of the Fjerritslev Formation of the Norwegian-Danish Basin; they occur as erosional remnants in the Fiskebank Sub-basin (Hamar et al. 1983) and more extensively in the Danish Sub-basin (Jensen et al. 1986). Over the remainder of the central North Sea area, including the mid-North Sea highs, Toarcian sediments are believed to be absent. However, this whole area was probably submerged and the absence of near-shore facies in the Cleveland Basin suggests that the sea had also transgressed over much of the Pennine High. To the south of the mid North Sea highs sediments occur in both the British and Dutch sectors, though they have been eroded away from the intervening area. The distribution offshore from the English coast is based on the Lias subcrop of Fyfe el al.'s (1981, fig. 2) map, while the facies is generalized from Rhys (1974) and Kent (1980b). In the Dutch area the Toarcian is represented in the Werkendam Shale Formation (ARena Group): the generalized occurrence is based on the distribution of the Altena Group shown by van Wijke (1987, fig. 2). The Anglo-Brabant platform was probably emergent, though over eastern Kent the boundary is drawn further north than for the late Pliensbachian times to allow for the deposition of bifrons Zone clay and shale there. Mudrocks were widespread over the Paris Basin (M6gnien & M6gnien 1980), apparently extending almost to the Armorican coastline, and have been proved in wells 98/22-1 and 98/22-2 in the Channel Basin. In the Western Approaches Trough the Toarcian is represented by marls and marly clays in the Brittany Basin (wells B E L l and LIN l) and possibly by the Liassic sandstones resting on basement in well LET I in the South Channel Basin. It is missing from other explored parts of the Trough so the southwestern extent of the Cornubian island remains speculative. In the Celtic Sea and Cardigan Bay Basins mudrocks are widespread and have been assigned to the Stratton Formation (Millson 1987; type well: Mochras). This occurrence, together with similar mudrocks in the Bristol Channel and Gloucestershire, gives no hint of the presence of a South Wales landmass. In western Scotland evidence that erosion of land areas was not as active as in the Upper Pliensbachian is suggested by the presence of Lower Toarcian shales on Skye, Raasay and the Shaint Isles, as well as by the Lower Toarcian Raasay Ironstone. A little further north, BGS borehole 72/33 cored a black mudstone of Pliensbachian or Toarcian age. Later in the Toarcian regression or renewed uplift of land must have taken place to provide the arenaceous deposits of the Bridport Sands, Yeovil Sands, Midford Sands and Cotswold Sands of various dates in west and southwest England. A similar event in the Midlands and eastern England is suggested by the late Lower Jurassic erosion that removed all post-midbifrons Zone deposits from a large area from Oxfordshire to Yorkshire before deposition of Aalenian sediments. Further north, delta front sands prograded over the Inner Moray Firth (Linsley et al. 1980) and Upper Toarcian sands in the north Viking Graben may also indicate shallowing (Eynon 1981). MKH MID JURASSIC By the beginning of the Aalenian there was a radical change in palaeogeography, caused by a fall in sea level in latest Toarcian times accompanied by domal upwarping with accompanying vulcanicity in the central North Sea basin. There is good facies evidence for Eynon's (1981) suggestion that severe upwarp commenced in the late Toarcian, and the effect of the dome is demonstrated both by an extensive area within which Lower Jurassic sediments are virtually absent (Maps J1-3), and by the broad concentricity of Middle Jurassic facies belts (e.g. Eynon 1981, fig. 3). The dome began to subside during the Mid or Late Bathonian; at about the same time volcanicity ceased (Dixon et al. 1981; Ziegler 1981, 1982). The upwarping and associated vulcanicity may have been due to lithospheric stretching rather than mantle upwelling (Dixon et al. 1981). The landscape relief was not as severe as today, but must have been
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sufficient at times to provide substantial volumes of sediment. Judging by their influence on sedimentation patterns, areas of highest relief were probably the Scottish highlands, the Scandinavian landmass and also the flanks of the Central North Sea Rift. If these areas reached 500 m above sea level, then the heights of more subdued features such as the AngloBrabant and Welsh landmasses may have rarely exceeded 150 m throughout the period. The paucity of large land areas in the British area on which major river systems could develop suggests that in many cases small neighbouring streams constructed fringes of coalescent deltas, so building out coastal plains with broadly embayed coastlines. The landmasses were surrounded by shallow epeiric seas. There was some tidal activity, which occasionally exceeded the microtidal/mesotidal boundary (Morton 1983). There is also evidence of storm activity, and with only modest to weak tidal currents, storm surge ebb currents must have assumed major importance in transporting nearshore sands into the offshore environment. An Aalenian-Bajocian northward spread of carbonate deposition into the British area was coincident with southward spread of similar environments in the southern hemisphere, and indicates climatic warming (Ager 1975). The Callovian shows a distinct reversal of this trend (Ager 1975), whereas the major eustatic sea-level rise at this time ought, by drowning terrigenous clastic source areas, to have encourged a continued expansion of limestone deposition. Middle Jurassic rocks probably cover only about 25% of the map area. They are largely or entirely missing from many of the western basins and the only high to have retained a substantial record is the Anglo-Brabant massif; thus the palaeogeography is open to varying interpretations. The Shetland Platform was emergent, providing sediment to the West Shetland and Unst Basins and north Viking Graben (e.g. Richards et al. 1988). For much of its length the Viking Graben appears to have been an alluvial basin, dispersing sediment northwards. Initially, sandy low sinuosity and braided streams with perhaps little net accumulation may have occupied the graben floor. As sea level rose and relief reduced, aggradation occurred and meandering streams developed; the Bathonian of the Sleipner Field consists of mudstones and siltstones with thick coals but only minor sandstones (Larsen & Jarvik 1981), suggesting stable meander belts within floodplains with only modest topstratum depositional rates. A similar change through time has been concluded for parts of the southwarddraining Central Graben (Koch 1983). At the junction of the Central Graben, Viking Graben and Moray Firth Basin a large volcanic centre was initiated (Howitt et al. 1975). Lava flows up to 9 m thick were repeatedly erupted onto a block-faulted terrain to form an extensive lava plateau covering at least 12000 km 2. Marine magnetic interpretations suggest that the volcanics (Rattray Formation) may be 30004000 m thick between the Forties and Piper Fields (Woodhall & Knox 1979). They were erupted subaerially, probably as fissure eruptions, though a particularly large magnetic anomaly adjacent to the Jaeren High suggests a basic igneous intrusion and a possible volcanic cone. The elevated lava plateau probably formed a watershed between basins to the north, south and west. Around the northwestern margin of the plateau, a fringe of interbedded lavas, tufts and alluvium derived from the lavas is preserved in the outer Moray Firth Basin. It grades laterally into dominantly terrigenous clastic sequences of carbonaceous mudstones and siltstones with sporadic sandstones and coals and a few thin tufts (Pentland Formation). However, a flaw in some Aalenian-Bathonian palaeogeographic maps (e.g. Ziegler 1982, encl. 19) is that the whole Moray Firth Basin is portrayed as an alluvial rift fill whereas palynofacies Studies in the Beatrice area indicates marine influence in the Inner Basin at various times (Linsley et al. 1980; M. J. Fisher, pers. comm. 1983). In view of the continental laval plateau to the east, marine access must have been from the west, probably through the basins between the Shetlands and the mainland (cf. Eynon 1981). The positive elements of the Vestland Arch were source areas. Further south an important connection of the Norwegian-Danish Basin with the Central Graben via a trough between the Mandal and Rinkobing-Fyn Highs is revealed by Bryne Formation isopachytes (Hamar et al. 1983, fig. 3c; Koch 1983, fig. 9). The formation consists of fluvial sandstones with intercalated siltstones, shales and coals of Aalenian to Bathonian age (Vollset & Dor6 1984). There may have been another connection between the Jaeren and Mandal Highs, at least during the Callovian. Norwegian views (Skarpnes et al. 1980; Hamar et al. 1983) that the Central Graben and Norwegian-Danish Basin formed a unified depositional basin during the early Middle Jurassic reflects their surprising belief that the Central Graben was not initiated until the late Middle Jurassic. Lower Triassic sandstones were the main source of siliciclastic debris on the Mid North Sea High (Thomas 1975). There was also significant siliciclastic supply to the East Midlands Shelf and Cleveland Basin from the Pennine High. The Anglo-Brabant Landmass, albeit subdued, was another important source of sediment. The Paris, Wessex and Channel basins were depositional areas. Paris Basin facies in Maps J4a-8 are mainly derived from M+gnien & M6gnien (1980). In the Wessex Basin it is clear that various Jurassic formations thin westward towards the Cornubian Massif, which was a source of sands at times (e.g. Corbin 1980). Both the Western Approaches and Celtic Sea basins were apparently fully marine and opened southwestward into the Bay of Biscay Trough. No sediments are preserved along the axial region of the Western Approaches Trough but there is a thick sequence in the Brittany
Trough. The Welsh Massif probably formed a strong positive feature, often emergent and providing sediment to adjacent areas. A particular problem is the extent of any Middle Jurassic seaway between the Irish and Scottish landmasses, for this has a direct bearing on the palaeogeography of the Irish Sea. Probably the greatest obstacle would have been the Islay-Donegal Platform, an extension of the northeast-southwest belt of Moinian-Dalradian metasediments along and off the northwest Irish coast. The Midland Valley Graben developed in the Devonian and subsided rapidly through the Carboniferous (George 1960). The younging of the later graben fill towards its southwest end indicates a post-Palaeozoic tilt of the graben floor in this direction, and suggests that the graben formed a shallow conduit for sediment from surrounding higher lands, transporting it into the Arran Basin where it further hampered any marine seaway through the North Channel. A shallow north-south seaway probably existed throughout the Middle Jurassic to the west of Britain and Ireland along the RockalI-Faeroes rifts. In the Slyne Trough, well 27/13-1 penetrated shallow marine and nonmarine clastics of Aalenian to early Callovian age; facies are similar to those in the Hebrides Basin. In the latter area the large-scale cross-bedded facies of the Bearreraig Sandstone Formation (Aalenian-Bajocian) indicates strong tidal currents through a narrow seaway whose eastern coastline probably lay close to the present mainland coast (cf. Hudson 1964). The seaway closed temporarily during the Bathonian. In the Faeroe Rift, thick units of poorly sorted sandstone suggest sediment gravity flows, probably triggered by fault movements along the Rona Ridge. This ridge may well have been emergent or the site of shallows at times, when it protected the West Shetland Basin from strong wave and current activity. Sandstones predominate in the basin-fill (Hitchen & Ritchie 1987), accompanied by coals and limestones with shallow marine shelly faunas, reflecting an interplay of various coastal environments. Jurassic vulcanicity reached a peak in the Middle Jurassic. In addition to the lava plateau discussed above, subsidiary Middle Jurassic volcanic centres have been identified in the Egersund Sub-basin, the north Viking Graben, the south Viking Graben, western Norway and southern Sweden (e.g. Dixon et al. 1981; Ziegler 1982). The volcanics are all of moderately to strongly undersaturated alkaline character. Igneous activity also occurred to the west of Britain. Bentonites in the Upper Bathonian of the Bath area (Jeans et ai. 1977) probably reflect vulcanicity along the margins of the Porcupine Basin or Biscay Rift, for mid Jurassic basaltic andesites occur on the Goban Spur (Cook 1987). Intrusive rocks are known in the Fastnet Basin (Caston et al. 1981). Volcanoes probably occurred along the margins of the Rockail-Faeroe Trough to the west of the Hebrides to form the source of ash transported into the Skye area (Knox 1977). In the map descriptions below, the names and correlation of lithostratigraphic units mainly follow Cope et al. (1980b). Correlation of the Bathonian is a particular difficulty owing to the wide spread of coastal and shallow marine environments of restricted circulation during that age (and consequent rarity of ammonites). The relative lateral uniformity of Bathonian environments in southern England has enabled the erection and successful application of zonal schemes using benthic, highly facies-bound fossils. Unfortunately, these schemes break down in the diverse facies further north. In central and eastern England, an event stratigraphy of the Upper Bajocian and Bathonian based on sedimentary cyclothems agrees well with existing biostratigraphic data and is used to tentatively propose a time-correlation of successions in areas such as the Inner Hebrides, where biostratigraphic control is very poor. Correlation offshore is complicated not only by biostratigraphic limitations but by the persistent usage of Bajocian in its old sense, that is including the Aalenian. MJB, DWC
J4a: Early Aalenian An early Aalenian marine transgression over the East Shetland Basin deposited the Broom Formation. The basal unconformity also extended over the Southern North Sea Basin where it may be precisely dated as basal s c i s s u m Subzone ( o p a l i n u m Zone). This provides the chosen time interval for the early Aalenian map. The unconformity increases in magnitude toward the central part of the Central North Sea, where the Lower Jurassic was extensively eroded following the late Toarcian doming. However, away from this area conformity not only occurs in major depocentres but at sites of very modest subsidence such as the South Dorset High. Globally, there is very little evidence for the rapid and substantial late Toarcian sea-level fall followed by early Aalenian rise suggested by Vail & Todd (1981), who may have been too strongly influenced by North Sea data. There the unconformity may result largely from the regional doming alone. Uplift along the flanks of the Rockall-Faeroe Trough during the same phase of severe lithosphere stretching may largely explain the major basal Middle Jurassic unconformity here also. In the East Shetland Basin the Broom Formation forms a sheet of coarse grained and pebbly feldspathic sandstones (Morton & Humphreys 1983). It shales eastward and presumably northward into the basin centres, but against the Shetland Platform has been interpreted by Eynon (1981) as a cliff-base beach-shoreface accumulation. Further east, fan deltas prograded westward over the northern Horda Platform to deposit the Oseberg Formation (Graue et al. 1987).
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South of the Mid North Sea High, primary iron-rich sediments accumulated in a shallow transgressing sea. Variably sideritic chamosite-rich sandstone and chamosite oolite (Dogger Formation) occupy much of the Cleveland Basin, where oolitic and bioclastic limestones fringing the extreme western and northern outcrop were taken by Hemmingway (1974) to indicate a nearby coastline. The Northampton Sand Ironstone of the east Midlands consists predominantly of chamosite oolite with siderite mudstones. The sandy nature of the stone along the western and southeastern margins of the ironstone field suggests the proximity of coastlines. Intercalations of rootletted claystone in south Lincolnshire and Cambridgeshire (e.g. Horton et al. 1974) indicate an at least locally swampy, possibly barrierlagoon transgressing coast. In the Severn Basin, the s c i s s u m Beds consist of sandy and sill~ylimestones overlain by shallow-marine bioclastic wackestones. Mudge (1978) postulated intertidal environments but belemnites and occasional ammonites indicate a subtidal origin. Shallow-marine sandstones, siltstones and sandy, bioclastic carbonates were deposited on the Cotswold-Weald Shelf, with terrigenous sediment here probably derived from both the Anglo-Brabant and ParisPlage areas. Further west, Cornubia was the main source of the clastic sediment in the very sandy s c i s s u m Bed. Coccolith-bearing Aalenian to Bajocian mudstones and limestones in the Bristol Channel Basin (Warrington & Owens 1977) suggest a bottom environment below normal wave base. During the early Aalenian, sands accumulated in the St George's Channel Basin; they form the lower part of the Galley Head Formation (Millson 1987). The extent of sea further north is speculative, but as a consequence of the relative lowering of sea level in the British area during the late Toarcian and early Aalenian the seaway between the Irish and Scottish Highlands Massifs is likely to have been closed. Paralic environments probably occupied the southwest Scottish and Ulster basins. In the Skye area strong tidal currents were confined to southern Strathaird. Variably muddy sands colonized by a diverse suspension-feeding fauna were deposited in moderately turbulent conditions to the north and on Raasay, whilst muds accumulated in deeper water farther offshore on Trotternish. In the Inner Moray Firth delta front progradation continued from the Toarcian, to deposit shales and sandstones (Linsley et al. 1980). Sediments in the West Shetland area are discussed in the Introduction. MJB, DWC
shoreface sands/siltstones followed by lower coastal plain deposits: interlayered and laminated siltstones and claystones, commonly organic-rich and rootletted, with broad channel-fills. This vertical facies sequence suggests coastal progradation over a shallow platform. Offshore, non-calcareous shales with subordinate siltstones and coals toward the base of the Middle Jurassic in the Sole Pit Trough are correlated with the Hayburn and Grantham Formations. Fully marine muds were being deposited to the southeast in the onshore and offshore south and central Netherlands. The Upper Aalenian is absent over the western flank of the AngloBrabant Platform due to late Bajocian erosion, but reappears in the Severn Basin as a laterally variable siliciclastic wedge in a limestone-dominated succession; non-marine palynofloras have been recorded from associated clays (Fenton 1980). Further south, the upper Aalenian is also absent across the Mendips Platform and the YeoviI-Cranborne and South Dorset Highs. Between swells accumulated thin limestones, typically containing abundant limonite ooids; the ooids may have formed in the very shallow water over the swells. Cephalopods suggest normal marine salinities. Cross-bedded, sandy, bioclastic limestones and silty, oolitic limestones accumulated on the Cotswold-Weald Shelf; an emergent ridge extending westwards from Paris-Plage may have been the source of the abundant siliciclastic sediment encountered in the upper Aalenian at Portsdown. In Normandy, immediately fringing the Armorican landmass, fairly condensed, cherty sediments and limonite--oolite wackestones were deposited, usually under normal marine salinities. Coastal plains backed by a wholly alluvial system may have filled the Irish Sea and pushed the late Aalenian coastline far to the south. Marine mudrocks in the North Celtic Sea Basin become sandy into the St George's Channel Basin (Galley Head Formation), and the reddened, apparently nonmarine mudstones encountered nearer St Tudwars Arch (BGS Borehole 71/55) may be of this age; a coastline probably fringed the arch. Marine regression continued in the Inner Moray Firth Basin. In the Beatrice Field, a coarsening upward shoreface sandstone sequence is followed by a claystone unit with rootlets, thin coals and marine microplankton, indicating saline bays/lagoons and swamps (Linsley et al. 1980). The coastline may have been pushed out of the Moray Firth Basin entirely into the area of the Orkneys, and marine regression could well have featured also in the West Shetland Basin, but a fully marine seaway with marked tidality persisted to the south and east through the Hebrides Basin, where limestones and silty limestones accumulated. MJB, DWC
J5: Early Bajocian J4b: Late Aalenian The late Aalenian in the British area was a time of marine regression. The most likely explanation for regression in the North Sea region is a high terrigenous sediment influx following domal upwarp, during a period of eustatic sea-level still stand or perhaps very slow fall. Downcutting of upper Hayburn Formation channels through the lower Aalenian around the margins of the Cleveland Basin (Livera & Leeder 1981) and similar features on the East Midlands Shelf (Hollingworth & Taylor 1951) argue for a sharp fall of sea level at the very close of the regression, whilst pre-Bajocian erosion of Aalenian strata on the East Midlands Shelf and in the Cotswold Basin may also have been a lowstand event. The map portrays onshore facies of the mid c o n c a v u m Zone ( c o n c a v u m / f o r m o s u m Subzone boundary). Whilst volcanicity continued in the Outer Moray Firth Basin, marine regression pushed the late Aalenian coastline in the north Viking Graben northward. In the Brent Province, marine sandstones or mudstones are followed everywhere by a coarsening-upward sand sheet comprising the Rannock and Etive Formations. Most workers (e.g. Eynon 1981; Brown et al. 1987) agree that the fine-grained micaceous Rannock sandstones were deposited in a nearshore to shoreface setting, but interpretations of the coarse-grained and locally cross-bedded Etive Formation include a distibutary channel system (e.g. Simpson & Whitley 1981), coalesced mouth bars (Albright et al. 1980), and the upper shoreface, beach ridge, barrier top and inlet channel deposits of a coastal barrier system (Budding & Inglin 1981; Hallett 1981). These interpretations may all be correct for the particular fields they address. Overlying sandstones and carbonaceous shales with coals and rootlets (Ness Formation) were deposited in an environment of marshes, bays, channels and lagoons (e.g. Parry et al. 1981; Simpson & Whitley 1981). The northward progradation of a major delta system is supported by most workers and the continuity of the delta front sandstones suggests a wave-dominated delta front. Evidence for a luxuriant vegetation and saline influence in bay, lagoon and channel subenvironments further suggests a tidally influenced subtropical delta model. Southward from the Mid North Sea High, a coastal plain built out into the Cleveland Basin. Rivers draining the Mid North Sea High and the north Pennine High deposited levee and overbank muds and silts and crevassesplay sands cut by the sandy fills of low-sinuosity channels (Hayburn Formation: Hancock & Fisher 1981; Livera & Leeder 1981). In the northern and northwestern parts of the basin, freshwater lakes up to 4 km in length were fringed by a horsetail-dominated flora (Hemingway 1974); thin coals accumulated locally. To the southeast, palynofacies analysis suggests brackish swamps drained by tidally-influenced channels (Hancock & Fisher 1981), whilst a transient marine incursion is indicated in the extreme southern part of the basin (Gaunt et al. 1880). Further south on the onshore part of the East Midlands Shelf, the Grantham Formation consists of bioturbated
Over the southern half of England and in the Western Isles, the map portrays facies of the ovalis Subzone of the s a u z e i Zone. Elsewhere it is more generalized. Across much of eastern and central England there was an early Bajocian marine transgression, which reflects a major eustatic sea-level rise (Hallam 1978). Early Bajocian facies in the British area therefore provide an excellent illustration of the effect of local tectonism (in this case Central North Sea doming) on eustatic sea-level rise. In the East Shetland Basin and northern Viking Graben the delta plain (Ness Formation) established during the Aalenian continued and was subject to marine incursions, some of which may be attributed to differential subsidence of the delta top due to syndepositional fault block movements (see Brown et al. 1987). The formation may be represented also in the Unst Basin (Johns & Andrews 1985). In the Norwegian-Danish Basin and adjacent parts of the Central Graben there is an extensive area of predominantly arenaceous, fluvio-deltaic deposits (Bryne Formation: Voilset & Dor6 1984); their distribution is modified from Hamar et al. (1983, fig. 3c, 'Haidager Formation'). To the south of the Mid North Sea High another 'delta occupied the Cleveland Basin during the early Bajocian. According to Livera & Leeder (1981) it was a small-scale version of the Niger Delta (fluvial-tidal-wave interaction). Marine transgressions onto the delta plain were interspersed with phases of delta progradation. The first two transgressions came mainly from the south (Bate 1967; Livera & Leeder 1981) but ammonite evidence indicates that the final and most extensive one (Scarborough Formation: early Bajocian) came solely from the east along the basin axis (Bate 1965; Parsons 1977). This transgression also advanced onto the East Midlands Shelf, but any seaway across central England at this time was shallow and restricted (Bradshaw & Bate 1982). The lower Bajocian of the Sole Pit Trough presents a more marine aspect than in the Cleveland Basin, but contrary to Kent (1980b) and Hancock & Fisher (1981), records of coals betray the persistence of a coastal plain rrgime flanking the Mid North Sea High and in continuity with the 'delta' plain to the west. The successions encountered by drilling, which all lie close to the Dowsing Fault Zone, appear comparable in facies to those along the southern margin of the Cleveland Basin (e.g. Brown Moor Borehole; Gaunt et al. 1980). Shales become more important southeastward, suggesting that a gradation once existed into the fully marine mudrocks (topmost part of the Werkendam Shale Formation) of the south Netherlands. Marine transgression in eastern England may have commenced very late in the Aalenian c o n c a v u m Zone (Ashton 1980; Gaunt et al. 1980). Onlap, locally preceded by erosion, occurred around the margins of the East Midlands Shelf where the Lincolnshire Limestone or its equivalents rest on Lower Jurassic claystones. The clear evidence of increasing depositional energy through the Lincolnshire Limestone may signify improving water
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circulation and therefore agitation as seaways widened and the depositional basin broadened during the sea-level rise. Initial marine transgression was probably from the southwest Netherlands through the Sole Pit Trough, and a beach ridge, lagoon and tidal flat complex may well have separated eastern England from more open marine environments in central southern England. Lower Bajocian sediments are absent over the Anglo-Brabant Platform. In the Severn Basin, silty and sandy, bioclastic limestones occur, while to the southeast, shallow-marine oolitic limestones are known from the Cotswold Weald Shelf. The ovalis Subzone is unrepresented across the Doulting, Yeovil-Cranborne and South Dorset Highs, but between swells and on the Mendips Platform, diverse shallow-marine limestones were deposited. Marine limestones of varying facies are extensively developed in the Paris Basin, while clays and marls occur in the Brittany Trough (released wells). Scattered boreholes in the Celtic Sea Basins indicate fully marine clay and mud sedimentation. Marine shales occur in the centre of the St George's Channel Basin, but a high proportion of sandy oolitic calcarenites near the margin in 103/2-1 suggests a fringe of shallow water carbonates. A shallow seaway was probably established through to western Scotland in the early Bajocian. In the north, strong tidal currents flowed close to the east coast of an Inner Hebridean strait, where they were deflected by shallow offshore banks formed by the fault-bounded mid-Skye High and a further basement high covering north Raasay and Rona (Morton 1983). Large-scale cross-bedded bioclastic sands accumulated in rapidly subsiding 'channels' between these highs. A lower-energy environment occurred farther offshore in Trotternish. Rootlets in the sauzei Zone at Torvaig (south Trotternish) suggest emergence of the offshore highs; interestingly this emergence coincided with regression in eastern England and hardground formation .in the Wessex Basin. In the Inner Moray Firth Basin, spore-rich clays accumulated in open pools on the floodplain, and Barnard & Cooper (1981) speculated that algal oil-shales may be present as a possible source of oil for the Beatrice Field. MJB, DWC
J6a: Late Bajocian There is a widespread unconformity beneath upper Bajocian strata from southern to eastern England. The major transgressive/deepening event appears to be of garantiana Zone age, and the map portrays facies of the acris Subzone of that zone. The severe erosion and truncation of folded and tilted strata during unconformity development across central and eastern England, together with the subsequent heavy influx of siliciclastic sediments, suggests that late Bajocian eustatic sea-level rise may have been initially matched or even exceeded in parts of the British area by epeirogenic upwarp. The pattern of the late Bajocian unconformity suggests a southward movement of the locus of Central North Sea domal upwarp to the vicinity of the Mid North Sea High. Late Bajocian erosion over the high is likely to have been very severe. In the Central Graben, the alluvial rrgime which characterized the Aalenian and early Bajocian probably continued, with alluvial plains passing into deltaic or coastal plains around the northern Texel-Ijsselmeer High. Whilst vulcanicity continued apparently unabated in the Forties-Piper area, a major marine transgression commenced in the north Viking Graben. Most interpretations of the well sorted and commonly large-scale cross-bedded Tarbert Formation sandstones agree on a shallow nearshore marine depositional environment (e.g. Hancock & Fisher 1981; Brown et al. 1987). To the south of the Mid North Sea High, sedimentation in the Cleveland Basin may have resumed already, following Iowstand erosion. Rivers draining from the northwest and north deposited a sheet of braided channel alluvium known as Moor Grit. Acanthomorph acritarchs in overbank shales at the top of this unit on the Yorkshire coast betray the presence of a shallow marine gulf in the Sole Pit Trough, where the offshore equivalent of the Scalby Formation/Great Oolite Group consists of 25-60 m of grey to grey-brown, commonly calcareous shales with siltstones, sandstones and argillaceous and/or sandy limestones. The fauna suggests a high proportion of marine sediments, but coals and lignitic layers indicate the occasional establishment of swamps. A southeastwardly increase in carbonate content reflects the wholly marine sequence of the southwest Netherlands. Much of England was emergent during the sea-level lowstand of the early late Bajocian, and the sea was probably confined to the central part of the Wessex Basin. On the East Midlands Shelf deposition recommenced in a large number of small lakes (up to 8m deep; most less than I km in length) occupying hollows on a perhaps karstic topography of Lincolnshire Limestone. Most of the lakes were filled with clay containing abundant plant debris, the others with sand. In southern England an erosive hardground developed during the lowstand, sedimentation resuming in the early garantiana Zone. There is an overstep of acris Subzone limestones towards the Cornubian, Welsh and Armorican Massifs, reflecting a quickening of sea-level rise in the late garantiana Zone; in the Mendips, they sit directly on Carboniferous Limestones and locally the acris Subzone is represented by conglomeratic limestones. In the eastern Weald, very sandy limestones were deposited upon Lower Jurassic sediments. These pass laterally westwards into argillaceous, bioclastic wackestones with subordinate intraclasts and ooids, in the central and western Weald. In southwest England, the acris Subzone Astarte Bed, a variably argillaceous, iimonitic bioclastic limestone, was deposited both
between and across swells. It passes northwards into the Upper Trigonia Grit and correlatives developed at the margins of the Mendips Platform. Both here and in the Severn Basin to the north, the oldest beds above the unconformity are shallow-marine, sandy, bioclastic calcarenites of the acris Subzone; these are overlapped eastward before the Moreton Swell by bioclastic, limonite=ooid wackestones of the parkinsoni Zone. To the northeast the late Bajocian unconformity cuts across tilted Jurassic strata onto Palaeozoic rocks over the Anglo-Brabant Platform and is locally rootletted. Marine clay and mud sedimentation continued along the axes of the Celtic Sea Basins, whilst shelly muds and thin sands were deposited in the St George's Channel Basin where circulation was probably more restricted. Limestones at the top of the Bajocian section in 103/2-1 suggest a possible carbonate fringe to the transgressing sea along the southern margin of the Welsh Massif. The seaway between Ulster and southwest Scotland, which may have closed during the sea-level iowstand, was probably re-established by the acris Subzone. In view of the conter/aporary palaeogeography of eastern and central England, alluvial systems occupying the Cheshire and Solway Basins may have fed coastal plains facing a transgressing sea to the west. In the Hebrides Basin, initial shallowing was ['oUowed by deepening/ transgression, with deposition of the richly ammonitiferous Garantiana Clay, which accumulated in moderate water depths well below wave-base. Attenuation of the clay and/or a lateral passage into argillaceous sandstones occurs as the basin shallows through northern Skye, but a marine connection with the North Minch Basin at this time is probable. MJB, DWC
J6b: Mid Bathonian Although the Bathonian was a time of major regression (Arkell 1956; Hallam 1978), Vail & Todd (1981) pointed out that the wide spread of regressive deposits argues for a stillstand or a slow fall of eustatic sea level less than the rate of subsidence for most shelves, hence maintaining a relative rise in sea level. Bathonian facies changes in the British area supports this conclusion. The map portrays onshore facies of the upper progracilis Zone. In the north Viking Graben a deepening event around the early/middle Bathonian boundary is indicated by the replacement of shallow marine (Tarbert) sandstones by more open marine calcareous mudstones (Heather Formation). Bioturbated siltstone with middle (Boreal) Bathonian ammonites in 9/10b-1 (Callomon 1975) demonstrate the southward penetration of fully marine waters, whilst further south vuicanism was probably drawing to a close'on the lava plateau in the Forties area. To the south of the Mid North Sea High, non-marine sedimentation continued in the Cleveland Basin for at least part of the Bathonian, though exact dating of the Scalby Formation is uncertain and depiction of facies on the mid- and late Bathonian maps therefore speculative. Early Bathonian rise in sea level caused lower valley gradients and higher channel sinuosities, greatly increasing flood sediment accretion (cf. Leeder & Nami 1979). Braided channel alluvium is succeeded by meander-belt sands and then more isolated channel sand bodies cut into mudstones, siltstones and occasional thin sandstones. Deposition was in a deltaic or coastal plain environment. On the East Midlands Shelf swamps built out during the early Bathonian; here and over adjacent parts of the Anglo-Brabant platform the remainder of the Bathonian saw the accumulation of a series of individually distinctive asymmetric cyclothems or rhythms, some composed mostly of terrigenous elastic sediments, others limestone-rich, but each initiated by marine transgression and completed by coastal progradation (Bradshaw 1978). They probably accumulated in a very shallow gulf or embayment experiencing lowered and/or fluctuating salinity and a humid climate. The map captures the palaeogeography during the regressive phase in the accumulation of the fourth Rutland Formation rhythm. Whilst the earlier marine transgressions onto the East Midlands Shelf were from the southwest with little eastward persistence, the fourth was more extensive, establishing a seaway between southern England and the Sole Pit Trough and permitting limestones briefly to accumulate by default, as it were, of terrigenous elastic sedimentation. A swamp environment probably persisted through the early and middle Bathonian on Humberside (Bradshaw & Penney 1982). In southern and central England, Lower Bathonian and older sands were redistributed by a mid Bathonian transgression, after which a belt of lime sand deposition became established along the Cotswold-Weald Shelf. Ooids formed in agitated waters where they were frequently moved as sand waves, dunes and ripples by currents generated by tides and winds, and wave action. Large shoals migrated rapidly during periodic storms. Palaeocurrent directions show some preferred northwest-southeast orientation which may reflect the basinal tidal flow pattern. A similar succession of sandy carbonates followed by purer oolitic lime sands accumulated in the Boulonnais; contemporary oolitic and bioclastic limestones are also present in Normandy, although there faunas are more diverse, reflecting the more normal marine salinities and lower physical stress which characterized the Armorican Shelf. At outcrop in the Cotswolds, shelf carbonates shale off into the slightly deeper-water, calcareous mudstones and argillaceous, fine-grained limestones of the Fuller's Earth facies developed in the southern Severn Basin. The same facies change occurred between Winchester and Portsdown, where oolitic lime sand was deposited at the edge of the Cotswold-Weald Shell and Fordingbridge and Arreton, where calcareous mudstones accumulated. Mudstones were deposited throughout southwest England and probably also across much of the
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Channel Basin; the carbonates of the Armorican Shelf are known to shale off into Fuller's Earth facies north of the Cotentin Peninsula. Although no volcanogenic sediments are indicated on the map, two possible active volcanoes are indicated on the margins of the Cornubian landmass, one in the Wolf Rock area and the other over the Lundy Horst: both sites are sources of later igneous activity which could be only part of a much longer history. Volcanoes in this area could be responsible for smectite in the progracilis Zone of the East Midlands (Bradshaw 1975). Fully marine clay and mud sedimentation apparently continued throughout the Bathonian in the centres of the Celtic Sea Basins. A facies similar to the Fuller's Earth Clay characterizes the northeastern Southern Celtic Sea Basins (Mizzen Head Formation) and the Bristol Channel Basin. Northeastward along the St George's Channel Basin, mud-shales become more silty and contain minor sandstones and limestones. In the Cardigan Bay Basin strata of probable mid Bathonian age are represented by BGS cores 71/57 and 71/50. Facies of oyster-rich and variably argillaceous wackestones and calcareous mudstones indeed resemble the Hampen Marly Formation and upper Rutland Formation of central England (Penn & Evans 1976). They suggest a protected setting, and the Cardigan Bay Basin probably formed a shallow gulf at the time. The seaway between Ulster and southwest Scotland was probably closed during the early to mid Bathonian, and the Slyne Trough had apparently become non-marine. Hudson (e.g. 1963; Hudson & Harris 1979) interpreted the Bathonian of the Inner Hebrides as having been deposited in extensive, partially protected shallow lagoons of low tidal range. The basin was bounded to the north by the newly emergent Rubh Reidh Ridge. Abandonment of the lower Bathonian Elgol Sandstone delta was followed by transgression with wave and current reworking of the delta front sediments (Harris 1984; Harris & Hudson 1980). The overlying Lealt Shale consists mainly of mud-shale, commonly thinly laminated and organic-rich. Faunas suggest abnormal and fluctuating salinities, mainly ranging between freshwater and brackishmarine (Hudson 1963, 1980). A narrow and probably fairly shallow portal to the Lealt Shale lagoon or gulf and a normally high run-off from the surrounding hills accounts for this condition. Bottom waters became poorly oxygenated at times and enabled the sedimentation of much organic carbon. The main water body was probably fringed by swamps and fiats with ponds; desiccation cracks and gypsum pseudomorphs occur in northern Skye and the persistence of a distinctive stromatolitic algal limestone with gypsum pseudomorphs throughout the Inner Hebridean outcrop (Hudson 1970; Harris & Hudson 1980) indicates an extended period of low hinterland runoff when at least the northern part of the gulf floor became a periodically exposed hypersaline mudflat. MJB, DWC
J7a: Late Bathonian An initial late Bathonian sea-level rise (hodsoni Zone) was followed by widespread shallowing and regression accompanied by a spread of terrigenous clastic sedimentation through the British area, suggesting a marked slowing in the rate of relative rise of sea level on at least a regional scale. The map represents the latter phase, portraying facies of the lower hollandi Subzone of the discus Zone. Fully marine clay and mud sedimentation continued through the late Bathonian in the Viking Graben but does not appear to have extended as far south as the Sleipner Field. In keeping with the rest of the British area, the graben probably experienced regression; coastal progradation across the Horda Platform is recorded by Brekke et al. (1981), but little data is available to assess the extent of this event elsewhere. Volcanicity in the Forties-Piper area had probably ceased by the late Bathonian, for a thin, largely argillaceous, coal-bearing unit overlies the voicanics and in the Sleipner Field its lateral equivalents are succeeded by lower Callovian sandstones (Larsen & Jarvik 1981). Facies in the Outer Moray Firth Basin and South Viking Graben suggest a floodplain with extensive swamps and perhaps lakes, and sluggish meandering streams. To the south of the Mid North Sea High, a fully marine environment in the Sole Pit Trough is suggested by the distribution and diversity of macrofauna and also microplankton (Fenton 1980), though a considerable withdrawal of the sea from its hodsoni Zone limits is likely. To the southwest, on the East Midlands Shelf, mudrocks (Blisworth Clay) with low diversity faunas indicate marine environments of restricted circulation. In the seaway linking this area to the Cotswold-Weald Shelf (Forest Marble), shoals become progressively more oolitic southwestward, whilst variably calcareous muds lacking many stenotopic faunal elements were deposited in a lagoon and tidal fiat complex on the Anglo-Brabant Platform to the east. Before hollandi Subzone times, the relatively high energy belt of lime sand marking the outer part of the Cotswold-Weald Shelf shifted southward by some 35 km to the Bath area, probably due to the reactivation of deepseated faults. Sharp facies changes across the shelf margin in this area suggest a relatively steep resultant foreslope. As differential subsidence between shelf and basin slowed during the hollandi Subzone, mobile sand shoals became widely established across the shelf and diverse, brachiopoddominated faunas (the 'Bradford Clay fauna') flourished. According to Allen & Kaye (1973), the shoals were elongated and many hundreds of metres across, had a maximum vertical relief of about 2 m and were flanked by calmer, deeper waters where muds accumulated. There is evidence of tidal and storm activity and shoal tops were periodically exposed. The overall tidal flow may have been northwest-southeast (data of Klein 1965), and the
northeast-southwest palaeocurrent pattern obtained by Allen & Kaye (1973) may reflect flow at the mouth of the seaway to the northeast. In the Bath area the shelf margin remained sharply defined in the early hollandi Subzone and lime sand deposition persisted there, although coral patch reefs developed locally. Bioclast ooid grainstones grade southwards to offshore marine calcareous mudstones which were deposited below normal wave base but probably in shallower depths than the underlying Frome and Upper Fuller's Earth Clays. This facies extends across the Channel Basin to intercalate with shelf carbonates around the Armorican Massif. In later hollandi Subzone times regression occurred across eastern and southern England. Marine clay and mud sedimentation apparently continued through the Late Bathonian of the Celtic Sea basins, but there is evidence of a major late Bathonian regression in the Irish Sea area. In the Cardigan Bay Basin, red mudstones and sandstones represent coastal plain environments. To the southwest, in the St George's Channel Basin, wells 107/2-1 and 106/24-1 proved red, yellow and grey-green non-marine to marginal marine mudstones at the top of otherwise wholly marine Bathonian sections, indicating eventual emergence of the entire area. Traces of anhydrite were reported from 107/2-1, which suggests that reddening occurred in a semi-arid climate. These red beds belong to the Schull Formation (Millson 1987). Red beds also cap the Bathonian sequence in the Inner Hebrides. Here the marine Duntulm Formation is followed by the variably calcareous mudstones of the Kilmaluag Formation. These apparently accumulated in very shallow pools largely isolated from the sea to the south, frequently drying up (Hudson 1963, 1980; Tan & Hudson 1971). The overlying Skudiburgh Formation comprises mainly siltstones and mudstones, mottled red, grey or greenish, and is apparently of alluvial origin. MJB, DWC J7b: Early Callovian The very latest Bathonian (discus Subzone) was marked by marine transgression and overstep through much of the area, depositing the Lower Cornbrash and correlatives. This event marked the beginning of the 'Callovian' eustatic cycle (Vail & Todd 1981; Haq et aL 1987). This sea-level rise probably occurred as a series of sharp pulses over some 2.5 Ma. By the beginning of the Callovian it had improved marine communications sufficiently to allow a migratory spread of kosmoceratid ammonite faunas from the Arctic and decrease microplankton provincialism (Fenton & Fisher 1978). An important pulse occurred during the calloviense Zone, and Map J7b portrays facies in the earliest part of that zone (early koenigi Subzone). Whilst recent reviewers agree that the most intense tectono-magmatic activity in the Central and Northern North Sea came earlier in the Middle Jurassic, major fault block movement occurred during the Callovian both here and in other parts of the British area (e.g. Ziegler 1981, 1982). In the Viking Graben many fault block crests were submerged; some, perhaps locally emergent, became sources of sand (e.g. Statfjord Field; Kirk 1980). Sands were also shed from the graben margins, where in places (e.g. in the vicinity of the Beryl Field) a fringe of well sorted shallow marine siliciclastics pass basinward into Heather Formation shales. At the southern end of the graben the Callovian of the Sieipner Field consists of three transgressive-regressive cyclothems of fine- to coarse-grained and pebbly sandstone with minor coal (Larsen & Jarvik 1981). The major marine transgression initiating the second and by far the thickest cyclothem may, in keeping with events in the nearby Scottish area to the west, have commenced in the early calloviense Zone. Though the Vestland Arch was the principal sediment source, palaeogeographic reconstructions based on lateral facies variation (e.g. Larsen & Jarvik 1981, fig. 14) suggest that the area to the south (at the junction of the three major North Sea basins) behaved as a high throughout the interval. Callovian strata have not been recorded from the Outer Moray Firth Basin (e.g. Harker et al. 1987). The northwestward extent of transgression through the Central Graben is unclear; while marine shales (Haugesund Formation) have been reported from the Norwegian Central Graben, Hamar et al. (1983, fig. 2) indicate that they are no older than late Callovian. Whilst the changing pattern of faunal provinciality during the early Callovian suggests improved marine connections between the Boreal and Tethyan Realms, this need not necessarily have involved an immediate reopening of a north-south seaway through the Central North Sea area. However, a narrow seaway was established between the Stord Basin and Fiskebank Sub-basin (via the Egersund Sub-basin) during the early Callovian. The Sandnes Formation here consists mainly of shallow marine sandstones (Hamar et al. 1983). A southward marine connection to the Danish Central Graben via a trough between the Mandal and RinkebingFyn Highs is also likely in view of its earlier Middle Jurassic history (Koch 1983, fig. 9), and seaways may well have been established later in the early Callovian between the Sele High and the Vestland Arch, and also across this arch to the south of the Jaeren High. A widespread erosional unconformity between the Lower and Upper Cornbrash in England suggests a possible relative fall in sea level at the Bathonian/Callovian boundary. Following this, variably argillaceous limestones (Upper Cornbrash) spread progressively northwards (Callomon 1955). Behind this advancing belt of carbonate sedimentation, clays were deposited in deepening water. Offshore, at the top of the West Sole Group, are 6-17 m of siitstones, sandstones and limestones with some shale, which correlate lithostratigraphically with the Cornbrash and Kellaways Beds onshore.
119
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JURASSIC
A sheet of sand (Kellaways sands) then spread over eastern and southern England, mainly during the calloviense Zone. Such a facies change would ordinarily suggest regression (e.g. Hallam 1978). Nevertheless, most of the earlier major Middle Jurassic transgressions in the British area were associated with 'basinward' spreads of siliciclastic sand and the Kellaways sands are considered another example of this phenomenon. Coastal and alluvial sand bodies deposited during earlier, lower sea-level stands were reworked to be moved offshore during phases of strong shelf current activity (e.g. storms). The sands are locally transgressive, e.g. over the Market Weighton High. While sand deposition commenced as early as the kamptus Subzone (macrocephalus Zone) along the southern margin of the Anglo-Brabant landmass, Kellaways Clay deposition persisted into the koenigi Subzone in the southern Severn Basin and across southwest England. However, terrigenous sand even spread into these basins by the calloviense Subzone. With the close of the caIloviense Subzone, mud deposition (Oxford Clay Formation) resumed over much of the East Midlands Shelf, though not in the north toward the Market Weighton High, where shallow marine sands persisted. Inversion commenced in the Cleveland Basin. The Kellaways sand of the Weald coarsens toward the Anglo-Brabant Landmass; it continued accumulating along the northern margin of the Weald Basin into the early jason Zone, 1 Ma after a return to muds some 80 km to the southwest. In the Celtic Sea area the Dungarvan Formation is predominantly argillaceous and of limited areal distribution (Millson 1987). Faunal indications of relative rise in sea level in the Inner Hebrides during the macrocephalus Zone coupled with evidence for marine transgression in the St George's Channel Basin indicate an early Callovian re-establishment of the seaway between Ireland and Britain. The lower Callovian sequences exposed on both sides of northern Scotland have been interpreted by Sykes (1975a,b) as the result of barrier-lagoon transgressions; major koenigi Subzone sea-level rise drowned the barriers and redistributed the barrier tops to form sublittoral marine transgressive sands. MJB, DWC
J8: M i d C a l l o v i a n Where present, facies of the uppermost calloviense Zone (enodatum Subzone), jason Zone and lower coronatum Zone (obductum Subzone) are generally similar to those of the upper coronatum Zone (grossouvrei Subzone), which Map J8 portrays, but the onshore record of the earlier divisions is more sporadic due to hiatus in many areas. The mid Callovian is characterized regionally by organic-rich shales, the only extensive development of this facies within the Middle Jurassic of the British area. The lack of such sediments further afield suggests that the facies was controlled not by an oceanic anoxic event but by local rates of sea-floor subsidence. The propagation and enforcement of a stable density stratification was facilitated by a pattern of shallow sills, created and sustained in at least some cases by tectonic activity, which inhibited the recycling of bottom waters in broad intervening lows. As transgression continued and highs (especially the Market Weighton High) were onlapped, bottom water circulation improved until by early in the late Callovian (athleta Zone) organic-rich mudrocks had given way to organic-poor and calcareous ones. In the East Shetland Basin and north Viking Graben a calcereous mudstone facies predominates (Heather Formation), though shallow marine sand bodies border the graben margins in places. The distribution of the Heather Formation is generalized from published accounts of individual oilfields and released wells. It may exaggerate the areal extent of Middle Callovian sediments, which are often absent over intra-graben highs, due largely to persistent wave-winnowing during a phase of active fault block movement. In the Troll Field deposition of the main reservoir sand, the Sognefjord Formation, may have started by this time (Hellem et al. 1986, figs 17, 18) though the main body of sand is Oxfordian to Kimmeridgian. It represents a marine coastal offlap sequence derived from the Scandinavian landmass to the east. In the Unst Basin, Heather Formation siltstones accumulated; their generalized distribution is based on data in Johns & Andrews (1985). Coastal progradation in the Sleipner area to complete the second Hugin Formation cycle (Larsen & Jarvik 1981) means that there would have been no seaway from the South Viking to northern Central Grabens at this time. But the Horda Platform was flooded to connect southward with the Norwegian Basin and adjacent parts of the Central Graben, Distribution of Sandnes Formation sands is generalized from Hamar et al. (1983, fig. 4a). There may be a mid Callovian hiatus in the Norwegian Central Graben, but this time interval is probably represented in the adjacent northern part of the Danish Central Graben by the higher part of the Bryne Formation (Bathonian Callovian), which consists of sandstones with siltstones, claystones and scattered thin coal seams. While the lower part is deltaic the upper beds are coastal lagoon and barrier island deposits, suggesting open marine conditions nearby. These upper beds are represented in the main part of the Danish and Dutch sectors of the graben by the Lower Graben Sand Formation, dated in the Dutch sector as mid to late Callovian (Herngreen & Wong 1989). The sediments were derived from adjacent highs and drainage was mainly to the south (Koch 1983); the initial environment was fluvial, passing upward into deltaic sediments with occasional marine intercalations. These apparently came from the south (Koch 1983), but the transgression which terminated non-marine sedimentation came from the north (Jensen et al. 1986), reaching the northern Danish sector in late Callovian times to deposit Lola Formation mudrocks.
To the south of the Mid North Sea and Texel Highs there was a continuous belt of marine deposition, though sediments are only patchily preserved. In the Dutch offshore area no Callovian strata are known and the extent of sea is conjectural. Onshore, the Brabant Formation is preserved as an erosional remnant in the Roer Valley Graben (NAM 1980). The map portrays facies of the Upper Brabant Limestone Member. In the Cleveland Basin sands, locally with chamosite ooids, accumulated (Langdale Beds) and organic-rich mudstones are confined to the southernmost part of the area. This reflects early structural inversion of the Cleveland Basin which was uplifted, with gentle folding, during the jason Zone. Siliciclastic sediments also characterize the northwestern part of the Sole Pit Trough, but farther south muds and clays were deposited. The Lower Oxford Clay extends from eastern to southern England. Its dominant lithology is olive grey, organic-rich clay-shale, thinly laminated, especially in the grossouvrei Subzone. The clay onlaps the Anglo-Brabant Platform, where persistence of its fine-grained lithology with only very gentle thinning onto the platform points ,to extensive progressive transgression through the mid Callovian, though there is palaeontological evidence of land nearby. An abundance of smectite in Dorset (Morris 1980) may reflect weathering of Permian lavas on the Cornubian landmass, though a contemporary voicanogenic origin is also possible. Mid Callovian facies are more varied in the Paris Basin, where limestones are important (Mrgnien & Mrgnien 1980). Sediments of this age may also be represented by poorly dated marls and sandy clays in the Brittany Trough. Mudrocks in the Bristol Channel Basin (Lloyd et al. 1973) appear to be organic-rich and similar to the Oxford Clay onshore, but age-equivalent shales in the St George's Channel Basin contain less than 1% organic carbon (Barr et al. 1981). With continued sea-level rise to the west of Scotland, organogenic sedimentation commenced below wave base. At Stafiin in northeast Skye, ammonitiferous, organic-rich shales occupy virtually the entire Middle Callovian. The coronatum Zone shales are tuffaceous, as are those of the Upper Callovian, and the athleta Zone contains a thin bentonite. The total stratigraphic distribution of volcaniclastic debris on Skye suggested to Knox (1977) a phase of strong volcanic activity to the west of Britain, probably sited along the Rockall-Faeroe Trough, starting in the mid-Callovian and with maximum activity in the late Cailovian and early Oxfordian. In the Inner Moray Firth Basin the middle Callovian of the Beatrice Field is argillaceous and fully marine, as is the Middle Callovian at outcrop which is again organic (sapropel)-rich. Middle Jurassic sands in the West Shetland Basin probably include Callovian strata, while well 206/5- I, just within the Rockall Trough, proved 76 m of Callovian mudstones and minor sandstones representing submarine fan abandonment as faulting waned (Hitchen & Ritchie 1987). MJB, DWC LATE
JURASSIC
In general, sediments of late Jurassic age are more extensively preserved than earlier Jurassic ones, though again they occur mainly over the eastern part of the map area, Thus the extent of previously established landmasses is still speculative, though most probably shrank through the Oxfordian and Kimmeridgian in response to rising sea levels. However, late Jurassic tectonics in the Netherlands and Celtic Sea areas led to local emergence against the regional sea level trend; the changes are documented under the appropriate maps. During the Portlandian (Voigian) the regional late Cimmerian movements caused renewed rifting and block faulting. In the North Sea this created extensional half grabens which eventually provided the traps for many of the Jurassic oil fields. Coupled with the tectonism, the end Jurassic global fall in Sea level caused widespread emergence, leading eventually to the isolation of the southern part of the map area as a region of non-marine sedimentation. On a broader scale this sea-level fall led to near isolation of the Boreal Realm and its biota by the end of the Jurassic. Thus inter-regional correlation becomes difficult, so that even the Jurassic-Cretaceous boundary may be drawn at different levels in different regions. Alternative stage names have thus developed for the terminal Jurassic stage: Tithonian for Tethyan regions, Portlandian for the sub-boreal area of southern Britain and northern France, and Volgian for boreal areas. To complicate matters further, the Kimmeridgian and Portlandian stages have two very different interpretations. When d'Orbigny interpreted the Kimmeridgian stage and listed characteristic faunas, he stated its equivalence to the Weymouth Beds and Kimmeridge Clay of Fitton (1836), which coincide with the sequence now known as the Kimmeridge Clay Formation. British workers have thus defined the stage to embrace the baylei to fittoni Zones, a procedure followed here. Unfortunately d'Orbigny listed an ammonite which occurs in the middle part of the Kimmeridge Clay (Gravesia) as characteristic of the overlying Portlandian stage, and this is the interpretation that the French have followed. Thus in northern France the Kimmeridgian stage only embraces the Lower Kimmeridgian sensu anglico. The base of the Volgian stage of the Boreal Realm, as modified by Gerasimov & Mikhailov (1966), coincides approximately with the base of the Portlandian stage sensu gallico. The name Volgian has been widely used by oil companies in the northern North Sea, which means that the Kimmeridgian is often used there in its restricted sense. However, the ammonite faunas for at least the lower part of the North Sea 'Volgian" are actually typical British sub-boreal ('Kimmeridgian') forms, not boreal ones. JCWC, PFR
14
CAMBRIAN
Massif or on its margins. The Cambrian of Nuneaton accumulated to the southwest of this Charnian Massif. Coastal Hartshill Formation sandstones (cycle l, thick), through to condensed shallow water, shelly Hyolithes limestones (cycle 2, thin) and deeper water Mn-rich shales (cycle 3, thick) exhibit a trend reciprocal with that of Shropshire and close to that of the Avalon succession, SE Newfoundland. The nearby Lickey Quartzites are lithologically similar but are of uncertain age. The former presence of limestones is suggested by clasts in the Permian Nechells Breccia (Brasier & Hewitt 1981). The Withycombe Farm borehole near Banbury, Oxfordshire, gives an indication of conditions to the east of the Malverns. Volcanic basement of presumed late Precambrian age is there overlain by the Withycombe Formation, a relatively thick sequence of sandstones bearing trace fossils (cf. cycle l) passing up into a thick succession of offshore shales with Aldanella sp., Watsonella sp., Platysolenites antiquissimus, Orthotheca cylindrica and Lontova-type acritarchs (Rushton & Molyneux 1990). This invites comparison with the Chapel Island Formation (cycle 1) or lower Bonavista Formation (cycle 2) of southeast Newfoundland, both of attleborensis age. MDB
E~lb: Southern British Isles: St David's The St David's Series is equivalent to the Middle Cambrian of European usage, but not necessarily equivalent to that of other regions such as Australia, China or North America. The Series is subdivided according to the Swedish zonal scheme, with a lower, a middle and an upper part. In the Welsh Basin the lower part is represented mainly by thick turbiditic sandstone successions; in Shropshire by thinner glauconitic sandstones and mudstones, and in the English Midlands by mudstones. Biostratigraphical control is generally poor and correlation insecure (Cowie et al. 1972; Rushton 1974). Faunal control is better in the middle part of the Series, where the following biozones are used:
Ptychagnostus punctuosus Biozone, Hypagnostus parvifrons Biozone, Tomagnostus fissus Biozone, Ptychagnostus gibbus Biozone. The gibbus Biozone appears to represent a widespread transgression, possibly related to a eustatic sea-level rise (Rowell et al. 1982, p 176), but that zone is not easily recognized in Britain. The overlyingfissus Biozone is more widely recognized and this interval, for which correlation is more secure, is represented on the map. In the Shropshire area glauconitic sandstones and calcareous beds were deposited in shallow water, and Cobbold (1927) demonstrated a local unconformity indicative of uplift along the Church Stretton fault zone at about this time. The absence of the St David's Series in the Malvern and Lilleshall areas (Rushton et al. 1988) implies similar uplift, but with less certainty. The Midlands Platform carries a thin sequence of variegated mudstones deposited below wave-base (Abbey Shales, three biozones represented, < 40 m thick in total). The fauna is dominated by agnostid trilobites and small brachiopods. Similar but thicker successions (c. 200 m total?) occur in southwest Wales and also in North Wales, though there the mudstone sequence is underlain by turbiditic sandstones offissus age (the Gamlan and Caered formations) that show evidence of derivation from a southerly source (Crimes 1970). This implies a positive area to supply sediment to north Wales whilst by-passing the St David's area in the southwest. The presence of bentonite beds indicates contemporaneous volcanic activity, though the location of the activity is unknown. There is no evidence for the St David's Series north of the Harlech Dome and St Tudwal's Peninsula, and it is assumed that the Irish Sea Horst was a positive area; nevertheless, no contemporaneous sediments are known to have been derived thence. The non-agnostid trilobites most typical of this epoch, Paradoxides and blind benthic forms (Hartshillia, Holocephalina) characterize outer-shelf environments in Wales and elsewhere in Avalonia (southeast Newfoundland, New Brunswick) and in Baltica and the margins of Gondwana (Conway Morris & Rushton 1988). After fissus Biozone times, mudstone deposition prevailed (except locally in the Comley area of Shropshire). There is a widespread hiatus after punctuosus Biozone times and before the Merioneth Epoch, the mudstone succession in the Nuneaton district being the best constrained (Taylor & Rushton 1972). In southeast Ireland the marginal basin of Leinster has yielded some evidence of the St David's Series (Smith 1977, 1981). In Co Wexford (the Cahore and Ribband Groups) and Co Wicklow (the Bray Group), the sediments were deposited in deep water (Crimes & Crossley 1968). These rocks yield trace-fossils, especially Oldhamia, in abundance (Dhonau & Holland 1974), but acritarchs, in part of St David's age, are the only bodyfossils known; the best preserved are from the Booley Bay Formation (Ribband Group) at Hook Head on the south coast (Smith 1981). Sediment was derived from both the southeast (Irish Sea Horst) and the northwest, where a positive area, possibly a marginal arc, is thought to have provided a source area. Less is known of the eastern Caledonide belt to the northeast of the Midland Platform and facing the Tornquist Sea. The clastic sedimentary
rocks encountered in boreholes in Lincolnshire (Cowie et al. 1972, table 2) are varied--phyllitic slates, feldspathic sandstones and greywackes--and a Cambrian age has been suggested, but without adequate faunal control. It is assumed here that this area was part of a marginal basin that existed throughout the early Lower Palaeozoic and extended from northern England into the Brabant area of Belgium where clastic rocks of Comley and St David's age are known (Vanguestaine 1974, 1978). It is not known whether the Precambrian of the Charnwood Forest area was emergent; there is some geophysical evidence to the northeast of Charnwood Forest for the presence of a post-Charnian, pre-Triassic succession that possibly represents part of the local Cambrian (Maguire 1987). A positive area in northern Norfolk was formerly postulated on the presence of supposedly Precambrian volcanic rocks encountered in the North Creak Borehole, but these are now thought to be part of the east Midlands Caledonian Belt (Pharaoh et al. 1987). AWAR
4~2a: Southern British Isles: Merioneth In Scandinavia the Merioneth Series has been divided into some eight biozones and thirty subzones, and this scheme can be applied in areas, such as England and Wales, where the olenid trilobite biofacies is developed. The interval depicted on the map is the Olenus Biozone and more particularly its upper part (O. cataractes Subzone). Deposits formed in England and Wales during the Merioneth Epoch reflect generally stable tectonic conditions. After a widespread non-sequence in the late St David's successions (possibly caused by a eustatic regression), a major new sedimentary cycle commenced, apparently before the end of the St David's Epoch, in both the Welsh Basin and the English Midlands (Rushton 1978; Allen et al. 1981). The Welsh Basin, though deeper at the beginning of the Merioneth Epoch, was partly filled towards the end of Olenus Biozone times with alterations of shales and contourite(?) sandstones (upper Maentwrog Formation), and those beds are overlain by thick shallow-water sandstones (Ffestiniog Flags Formation, >650m, Allen et al. 1981). There is some evidence in the Harlech Dome, St Tudwars Peninsula, and in the St David's area, that the sediments were deposited by currents flowing from the south, as in the St David's Series. To the north of the Harlech Dome, however, the Merioneth Series is comparatively attenuated and includes shallow-water quartzitic sandstones, with trace fossils (Cruziana facies) that appear to have been derived from the north, though the indices of current orientation give a wide range of directions (Crimes 1970). Muddy sediments accumulated over the site of the Midlands Platform and it is assumed here that these (Outwoods Formation of the Nuneaton Inlier, with known subsurface extension to the south and west) transgressed the Precambrian Charnwood Forest massif. The fauna of the Outwoods Formation includes olenid trilobites that tolerated oxygen-poor environments, together with a number of other trilobite genera of wide palaeogeographical distribution that inhabited outer-shelf regions (Rushton 1983). In the Welsh Borderland the Olenus Biozone is unrepresented, the earliest representatives of the Merioneth Series being the Orusia Shales (Parabolina spinulosa Biozone) resting unconformably on Comley and St David's rocks. However, at Malvern there is a thin sequence of Cyclotron-bearing shales that are thought to be of Olenus Biozone age. Uplift along the Church Stretton and Malvern lineaments is suggested by northeastwardly directed slumps or currents in the Moor Wood Flags Formation of the Nuneaton area (Taylor & Rushton 1972, p. 22). The derivation ofmicaceous detritus in the Llangynog area, south Wales, is believed to be from a metamorphic basement (Pretannia) to the south (Cope & Bassett 1987). The later Merioneth Epoch, from Parabolina spinulosa Biozone to Peltura scarabaeoides Biozone times, was dominated by deposition of black mudstones in disaerobic environments (Doigeilau, Bentleyford, Monks Park and White-Leaved-Oak formations). These pyritic, carbonaceous and uraniferous muds contain little or no coarse clastic material and are considered to represent a period of slow deposition; the proved olenid zones differ from locality to locality (Thomas et al..1984, p. 15), and the extent of possible breaks in deposition, comparable with those inferred in the contemporaneous rocks in Scandinavia, is not certain. The black mudstones in the Croft Borehole, Lilleshall (northern Shropshire), are unique in having many intercalations of current-bedded siltstone and sandstone, presumably of shallow-water origin (Rushton et al. 1988). The source of the silt-grade material is not known but is thought to originate from a local uplift along the Church Stretton Fault complex. The sedimentary rocks of the southeast Irish marginal basin have poor biostratigraphical control but include beds referable to the Merioneth Series (Ribband Group; Smith 1977, 1981). They are argillites and arenites that appear to have been deposited in deep water. The mudstones and finegrained sandstones of the Askingarren Formation, interpreted as underlying correlatives of the Ribband Group (Crimes & Crossley 1968, pp. 191,207), are considered as possibly late Cambrian in age. Flute casts indicate derivation from a southerly quarter. There is no evidence for the continued existence, into the Merioneth Epoch, of the marginal arc thought to have existed to the northwest of the southeast Irish marginal basin in St David's times. There is no biostratigraphical evidence for the Merioneth Series in the eastern English Caledonides (see the St David's Series, above). AWAR
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J9: Mid Oxfordian With sea levels rising through the Oxfordian, some of the land areas were shrinking, though tectonic movements in the Netherlands and the Celtic Sea area locally reversed this trend. Although mudrocks were widespread during the Oxfordian, limestones and sands of the Corallian facies spread over extensive areas by the end of the Lower Oxfordian, and the chosen time interval for this map, the tenuiserratum Zone, represents probably the maximum spread of the Corallian facies. By this time the Shetland Platform was at least partly submerged, and possibly completely so (Boote & Gustav 1987). To the north, Heather Formation siltstones occur in the Unst Basin (Johns & Andrews 1985). The same formation is widely distributed through the Viking Graben, mainly as a silty mudstone (Deagan & Scull 1977; Brown 1986), though limestone horizons occur in the northern part of the North Viking Graben. The formation may be locally absent over the Tampen Spur (Karlsson 1986). Further east, rivers probably flowed from the Scandinavian landmass along the line of the main present-day fjords (Oates, pers. comm.); one of them deposited a large wedge of sands to form the Troll field reservoir (Hellem et al. 1986). Silty mudstones also occur in the Norwegian-Danish Basin; their distribution is modified from Hamar et al.'s (1983, fig. 5b) map of the Egersund Formation. In the Outer Moray Firth mid Oxfordian marine and non-marine sands of the Sgiath Formation prograded northward (Boote & Gustav 1987) from the northeastern margin of the mid-North Sea High; the Fladen Ground Spur was not emergent at that time. Further west, another patch of sediments is preserved in the Inner Moray Firth. Here, nearshore silts of the Port-anRigh Member pass offshore into mudrocks, as in the Beatrice Field, but in some wells in Blocks 12/22, 12/23 and 12/30 a subtidal shoal developed, composed mainly of Rhaxella sponge spicules (Brown 1987, p. 787). Mid Oxfordian sediments occur through much of the Central Graben. They are predominantly marine but pass southward into northerly prograding non-marine sediments in the Dutch sector. In the northern part of the graben both mudrocks (Haugesund Formation) and sands (Fulmar and Ula Formations) occur (Hamar et al. 1983, figs. 4c, 5b). To the southeast, in the Danish sector, the Lola Formation consists of slightly calcareous claystones (Jensen et al. 1986). In the southern, Dutch part of the graben the geography has been interpreted from data in NAM (1980), Herngreen & de Boer (1985) and Herngreen & Wong (1989). Here the mid Oxfordian is represented by a regressive phase. The paralic Upper Graben Sand (late mid to late Oxfordian) in the area of well F3-3 separates the marine earliest Kimmeridge Clay to the north from the Puzzle Hole Formation, which consists of silty claystones and argillaceous siltstones. The latter Formation is mainly non-marine, but in well F11-2 the top beds contain late Oxfordian marine dinoflagellates. Further south it passes laterally into deltaic sediments of the Delftland Formation. In the adjacent Zuidwal Basin, the Zuidwal Volcanic Formation consists of basaltic breccias and tufts dated by NAM as probably Berriasian but by Perrot and van der Poel (1987) as 152 ___3, i.e. close to the Callovian/Oxfordian boundary. Herngreen & Wong (1989) suggest a link between the vulcanicity and the regression in the Dutch Central Graben. The palaeogeographical setting of the Central Graben sediments is complex and our interpretation differs considerably from that of previous authors. Hamar et al.'s (1983, fig. 5a) Oxfordian-Kimmeridgian map portrayed the northern Central Graben as an almost completely landlocked marine embayment, the Ula and Fulmar sands paralleling the shoreline and being derived by sub-aerial erosion of the adjacent southern Vestland Arch and Auk Platform respectively. The embayment was supposedly open only to the southeast--towards the southern part of the Central Graben. This is unlikely when in the Dutch Central Graben the non-marine Delfland and Puzzle Hole Formations were sandwiched between highs and prograded northward into a marine area--suggesting that the marine influence was from the north, which is supported by Jensen et al.'s (1986) view that in the intervening Danish sector the marine Lola Formation represents a southward transgression. An alternative interpretation which avoids these anomalies is that the Fulmar sands may represent submarine reworking from the Auk Platform, possibly some distance from any shoreline (Johnson et al. 1986), and a similar model for the Ula sands could apply. There may also have been shallow marine connection across or around the Montrose-Forties high to the south Viking Graben. This interpretation removes significant constraints and allows us to link the central and southern North Sea areas across the western part of the mid North Sea High much earlier than previously suggested (e.g. Ziegler 1982, p. 66, who indicated a Kimmeridgian connection). While the drainage direction in the southern Central Graben was to the south for much of Callovian time, it had been reversed by the mid Oxfordian. This reflects a southward shift of the focus of tectonic uplift (NAM 1980. p. 35) which by now was probably in the vicinity of the Texel High. The basins to the south (Roer Valley Graben, West Netherlands and Broad Fourteens Basins) may have been marine still, though the youngest preserved sediments (in the Roer Valley Graben) are probably of early Oxfordian age--the Oisterwijk Limestone Member (NAM 1980, p. 34). Here regional uplift climaxed during the later Oxfordian to emphasize the northwest-southeast orientated structures (Brown t986). Sediments are preserved in an extensive belt from the southern North Sea
across eastern England to the Weald and Channel Basins. Basin inversion took place during the early Oxfordian (Kent 1980a), deeper-water clays and siltstones (Ampthill Clay and West Walton beds) extending over the East Midlands Shelf southward as far as Bedfordshire while Corallian facies limestones (and at times sandstones) accumulated in the Cleveland and Sole Pit Basins. Further south the Corailian facies reappears over much of southern England, passing laterally into clays in the deeper parts of the Weald and Channel Basins. In Berkshire there is evidence of volcanic activity (Chowdhury 1982). Corallian facies limestones are also extensively developed in the northwest Paris Basin (M6gnien & M6gnien 1980). No sediments are preserved in released wells along the axial part of the Western Approaches Trough, but in the Brittany Trough Oxfordian limestones and marly clays occur in wells BELl and LINI. To the northwest an extensive land area appeared as a result of early-mid Oxfordian uplift in the Celtic Sea area, probably in response to rifting in the proto-Atlantic nearby (Millson 1987). Thus the Irish and Cornubian landmasses were linked (Naylor & Shannon 1982, p. 141, fig. 14; Millson 1987, fig. 7). The Fastnet and Celtic Sea Basins were either emergent or areas of non-marine sedimentation, though no mid Oxfordian sediments are preserved over most of the area. However, in the St George's Channel-Cardigan Bay region Oxfordian sediments proved in wells 106/24-1 and 107/21-1 (Barr et aL 1981) include the mid to late Oxfordian Timoleague Formation (Millson 1987), consisting of grey and red mudrocks, locally anhydritic. These are brackish to freshwater in type well 106/24-1. Sands of about the same age in the East Bristol Channel (above the Oxford Clay) are non-marine to transitional marine (Evans & Thomson 1979). The geography north of this area is mainly speculative but marine sands and silts occur on Skye, indicating a marine channel between the Hebrides Platform and Scottish Landmass. In the Solan and West Shetland Basins and on the margin of the Faeroe Rift Upper Jurassic mudstones with thin sandstones occur, which in well 206/5-1 are dated as Oxford to Volgian (Hitchen & Ritchie 1987). JCWC, PFR
Jl0: Late Kimmeridgian (early Volgian) The time depicted on the map, the wheatleyensis Subzone of the wheatleyensis Zone, lies in the lower half of the Upper Kimmeridge Clay of southern Britain and in the lower part of the Upper Kimmeridgian sensu anglico (see Cope et al. 1980b). It corresponds approximately to the pseudoscythica Zone of the Lower Volgian of Russia (Cope 1985). At this time mudrocks were very widely distributed, and despite a lack of lateral continuity sediments from the Channel to northern North Sea basins have been placed in the Kimmeridge Clay Formation. Usage of this term is becoming more restricted as new names are introduced for some of the discrete developments. From the southern North Sea to Dorset a series of sedimentary cycles is obvious in much of the Kimmeridge Clay. The complete cycle is of four units; it begins with a clay followed by a bituminous clay; this is succeeded by an oil shale and finally a coccolith limestone. Oschmann (1988) reinforces earlier views that the cycles represent progressive depletion in oxygen in the clays to total anoxia at the end of each cycle. He proposed a new model for Kimmeridge Clay sedimentation derived from palaeogeographicai and palaeoclimatic considerations and suggested that mass stratification of the water with anoxic bottom conditions was the result of currents and counter currents between the North Atlantic Shelf sea and the Arctic Ocean. Variation in the oxygenation of bottom waters was influenced by four different cycles with widely varying periodicity, which affected the counter current system. Water depth during deposition of these sapropelic layers is particularly contentious. Hallam (e.g. 1975) and Aigner (1980) suggested depths of less than 10 m to a few tens of metres, whilst Tyson et al. (1979) postulated several hundred metres. The latter would imply a major transgression inundating much of the land, for which there is little evidence. The palaeogeography inferred here is consistent with Oschmann's (1988) conclusion of sedimentation in a quiet environment with water depths of 50-100 m. Over much of the East Shetland Basin, Unst Basin and Viking Graben the Kimmeridgian is present in typical oil shale facies ('hot shales', characterized by their high gamma-ray values). Here the shales are generally deeply buried and mature, to form the source of much of the North Sea oil. They were assigned to the Draupne Formation by Vollset & Dor6 (1984); their distribution is based on numerous sources including Dor~ et al. (1985) and released wells. While these mudrocks were accumulating sands were eroded from bordering highs following local uplift; the most extensive source was the eastern margin of the Shetland Platform, especially in the south where the Fladen Ground spur was emergent (Boote & Gustav 1987, fig. 10). The sands accumulated as a series of submarine fans which now form important reservoirs, e.g. South Brae (Turner et al. 1987). Facies distributions in the south Viking Graben are modified from Harris & Fowler (1987, fig. 13), while those in the Norwegian-Danish Basin are based mainly on Hamar et al. (1983, figs 4c, 5c) and Dor~ et al. (1985). The mudrocks of the Borglum "unit" become kerogen-rich in the Fiskebank and Egersund sub-basins--the Tau Formation. Ula Formation sands accumulated along the northeastern flank of the south Vestland Arch, which may have been locally emergent. Kimmeridge Clay underlies much of the Moray Firth area. Outcrops of
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Kimmeridgian beds occur on land in the vicinity of Helmsdale, where the succession of boulder beds interbedded with lower horizons in the Kimmeridge Clay is interpreted as submarine fans produced by the action of tsunami over a submerged but active fault scarp. In the outer basin sands were shed from the Halibut Horst, especially into the adjacent Witch Ground Graben (Boote & Gustav 1987) where they form important reservoirs. In the Norwegian and Danish sectors of the Central graben mudrocks of the Farsund Formation are widespread; they are described as dark grey shales in the Norwegian sector and as claystones with thin beds of brownish dolomite or limestone in the Danish sector. Their distribution is modified from Hamar et al. (1983, figs 4c, 5c) and Jensen et al. (1986). Adjacent to the Auk Platform on the western margin of the graben the Fulmar Formation sands continued to accumulate. As with the Oxfordian map, we take a more 'submergent' view than Hamar et al. (1983) in our geographical reconstruction of the areas bordering the graben. Progressive inundation of the Mid North Sea High began with the Callovian transgression, but it was not until late Kimmeridgian (early Volgian) times that the area was largely submerged. Over the eastern half (essentially blocks 36-39) most released wells record Cretaceous resting on Permian or Triassic, but a few show Jurassic mudrocks of unspecified age. However, Fyfe et al. (1981, fig. 2) indicate an extensive development of Kimmeridgian and Volgian sediments. Hence we have not shown the facies distribution in this area but have indicated that some Volgian mudrocks are preserved. To the south of the mid-North Sea highs sediments are preserved in two main areas of the southern North Sea. Along the Dutch Central Graben the sea extended southwards from the Danish sector to deposit silty shales with thin dolomitic streaks, here assigned to the Kimmeridge Clay Formation. The shales become more silty and sandy southwards to pass into the predominantly deltaic Delfland Formation, which occupies the southern part of the Dutch Central Graben (Herngreen & Wong 1989). The Texel High was probably emergent. To the south, following the late Oxfordian uplift, the non-marine Delfland Formation started to accumulate early in the Kimmeridgian, its base marking an important regional unconformity. In both the Broad Fourteens and West Netherlands Basins it consists of sands and shales with some coal beds, interpreted as fluviatile and deltaic sediments. In the former basin it includes a local prodominantly nonmarine unit, the Fourteens Clay Member, which was originally interpreted by NAM (1980) to represent a marine interfinger from the north (Herngreen & Wong 1989). In the Central Netherlands Basin the Basal Weiteveen Clastic Member (conglomerates, sandstones and shales) of the Weiteveen Formation may be late Kimmeridgian. In the British sector the marine Kimmeridgian Clay Formation is widely distributed; its extent is generalized from Fyfe et al.'s (1981, fig. 2) and Cox et al.'s (1987) maps. The wheatleyensis Zone has been proved in BGS borehole 81/43 but is probably cut out in 47/15-1X, the 'type' well for the offshore area. From here the Kimmeridge Clay extends round the western margin of the Anglo-Brabant high to the Wessex and Channel Basins. Throughout this area the lithology is remarkably uniform, consisting of the cyclic sequence discussed above. Individual beds can be traced from BGS borehole 81/43 to the Dorset coast. Distributions are based mainly on data in Cope et al. (1980b), Smith & Curry (1975) and released wells. The sequence becomes more sandy in the northwestern part of the Paris Basin (M~gnien & Mrgnien 1980). In the Western Approaches Trough released wells indicate widespread absence of sediments, though marly mudrocks and limestones occur in the Brittany Trough (Wells BELl and LIN1). To the north of the emergent Cornubian Platform, 330m of typical Kimmeridge Clay occurs in the Bristol Channel Syncline (Evans & Thompson 1979) indicating normal marine conditions there. However, in the earliest Kimmeridgian the main Celtic Sea area apparently formed a shallow, predominantly non-marine embayment open only to the northeast. A late early Kimmeridgian marine inundation of the area deposited the mainly argillaceous Glandore Formation (late early Kimmeridgian-mid Portlandian: Millson 1987). The formation is not preserved in the southern Celtic Sea Basin, but in the centre of the northern basin and in Cardigan Bay it is represented mainly by grey, calcareous mudrocks low in organic matter (567 m in 106/24-1 and 57 m in 107/21-1). Along the northern margin of the northern basin is the sandy Leap Member. To the south of the Pembroke Ridge well 103/18-1 in the outer Bristol Channel shows 'regressive inner neritic sediments' of Kimmeridgian-Portlandian age (Naylor & Shannon 1982) suggesting that there could have been some land in the vicinity. Indeed, Millson (1987) indicated significant land areas over Wales in the early Kimmeridgian to Portland interval, but preserved sequences can equally be interpreted to suggest a smaller or even no Welsh Landmass, though it is unlikely that Wales received any great thickness of Kimmeridgian sediments (Cope 1984). Further to the west well 83/8-1, in the Fastnet Basin, shows a thick (433 m) brackish to freshwater carbonate succession of 'Kimmeridgian to Purbeckian (sic) age' (Robinson et al. 1981). Elsewhere in the basin 'Wealden' sediments rest unconformably on Toarcian or Aalenian strata (Ainsworth et al. 1987). These records support Millson (1987) in his view that land extended southwards from southwest Ireland by Portlandian times. On the map we interpret this evidence to suggest that shallower sediments close to land may be expected (where preserved) over much of the area of the western Celtic Sea Basin. There was probably an extensive area of shallow water between the Irish
and Scottish landmasses though no sediments are known until the vicinity of Skye. Here BGS borehole 71/8 penetrated Upper Jurassic mudrocks. Firmer dating comes from the Solan and West Shetland Basins, where mudrocks with minor sandstones are widespread (e.g. Hitchen & Ritchie 1987). JCWC, PFR
Jlla-d: Portlandian Although sea levels began to fall in late Kimmeridgian (mid Volgian) times the broad pattern of sedimentation over the North Sea area remained similar to that shown on Map J 10. For the Portlandian ( = later part of the Volgian) we have concentrated on the more rapidly changing geography and facies of southern Britain, illustrated by a sequence of four maps. Across eastern England the youngest preserved Kimmeridge Clay belongs to the pectinatus Zone and there is a major hiatus above, apparently representing a regional sea-level fall, though the area probably remained marine through the rest of the Jurassic (Rawson & Riley 1982). Sands spread over the East Midlands Shelf in mid Portlandian times (Spilsby and Sandringham Formations) while in the Cleveland Basin sedimentation only recommenced in the Ryazanian (Lower Cretaceous). The hiatus is marked by a phosphatic nodule band with remani6 ammonites indicative of most of the 'missing' zones. Initially a marine link was retained with southern England, where shallowing in late Kimmeridgian times was followed by silt and then carbonate sedimentation in the shelf seas of the Portlandian. Purbeck facies appeared and interrupted the marine record for the first time in the mid Portlandian as shallowing continued, with reversals, culminating in the termination of marine sedimentation to spread Purbeck facies over the whole area later in the later Portlandian (Townson 1975; Wimbledon 1985, 1987). Map J ll a portrays facies of the earliest Portlandian albani Zone. Marine seaways to the Bristol Channel and the Irish Sea apparently still persisted. Given the deeper water nature of the sediments, their uniformity, and their widespread ammonite faunas (France, Britain, offshore Denmark, Andoya and the USSR) it seems reasonable to assume that their absence in some areas is the product of later phases of erosion rather than non-deposition (Wimbledon 1985). Silty clays or argillaceous silts occur everywhere except over the presumed swell area of Portsdown (Taitt & Kent 1958). Between Swindon and Bedfordshire remani6 faunas of the albani zone are found as phosphatized internal moulds in the Upper Lydite Bed. They sometimes occur in a sandy limestone matrix, indicating marginal carbonate sedimentation adjacent to the Midlands landmass. The okusensis zone (Map J lib) marks the period of most varied facies in the Portlandian. In addition, clear faunal links make it possible to correlate very different sediments all around and across the Anglo-Paris Basin and in eastern England. In the deeper parts of the Wessex Basin primary dolomites were formed. In marginal areas sands, sandy limestg~nes and glauconite-rich sandy limestones predominate. Marine connectio0~ between the Pays de Bray, the Boulonnais, eastern England and the Wessex Basin are clearer than at any other time in the latest Jurassic. Seaways to the west and southwest, between Cornubia and Armorica, are possible but unproven from existing evidence. Eastern Cornubia was probably flanked by evaporitic environments of Purbeck type, which provided the Mg-rich brines which turned the lime muds of the deeper basin into dolomites. Wood and abundant plant debris are features of the shallow marine rocks of the okusensis zone/early kerberus zone in south Wiltshire, showing the proximity of land to the south, over a north Dorset swell, and/or to the north of the Mere Fault. By the end of kerberus Zone times the seas were already retreating from the Boulonnais and much of southern England. In north Wiltshire alternating palaeosols and marine limestones gave way to lagoonal micrites, channel infills and evaporites. The seaway to the North Sea was cut, and Purbeck facies spread to most of the area south of this former seaway. Only in the central part of the basin, in Dorset, Wiltshire, and possibly parts of the Weald, did normal marine environments persist. However, later erosion in the Weald, prior to the deposition of the Middle Purbeck Beds (sensu Dorset), limits our understanding of that region (Wimbledon & Hunt 1983). Faunas of the anguiformis zone (Map J1 lc) are restricted to ooid sands and bioclastic limestones of Wessex, the last unambiguously marine strata of the Jurassic of southern England (Wimbledon 1985). Some of the so-called Wealdien of the Boulonnais was deposited at about this time, although the overlying Wealden Ironsands there and in the English South Midlands (Swindon-Buckinghamshire) are probably later, of lower Middle Purbeck age. How long the presumed seaways to the Irish Sea and across central France lasted is not precisely known, but by the end of anguiformis zone times they had been cut and Purbeck facies spread over the entire area, though occasional marine incursions indicate the influence of nearby marine water bodies. JCWC, PFR, WAW Map J l l d shows the distribution of Purbeck facies and inferred geography for the Cypridea dunkeri Zone. The facies distributions are more generalized than on maps J I la-c. Correlation with the marine facies over the East Midlands Shelf is speculative but in the latter area sediments of the 'oppressus' Zone may be the approximate time equivalent. By now the late Cimmerian movements had clearly influenced the Wessex Basin. In particular, the South Dorset-Isle of Wight High was probably emergent. The basin was very restricted and evaporating at this time. Around the margins
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coniferous forests grew in a semi-arid, Mediterranean type of climate (Francis 1983). The southern extent of the basin is speculative but eastward connection to the Paris Basin was likely, while the connection to the Bristol Channel area had probably closed. PFR, I M W
References AGER, D. V. i975. The Jurassic World Ocean (with special reference to the North Atlantic) In: FINSTAD,K. G. & SELLEY,R. C. (eds) Jurassic Northern North Sea Symposium, Stavanger, September 1975. Norwegian Petroleum Society, Oslo, paper 1. AtGNER,T. 1980. Biofabrics and stratinomy of the Lower Kimmeridge Clay (Upper Jurassic, Dorset, England). Neues Jahrbuch fiir Geologie und Paltiontologie. Abhandlungen, 159, 324-388. AINSWORTH,N. R., O'NEILL,M. M., CLAYTON,G., HORTON,N. F. & PENNY,R. A. 1987. Biostratigraphy of the Lower Cretaceous, Jurassic and Late Triassic of the North Celtic Sea and Fastnet Basins. In: BROOKS,J. & GLENNIE, K. W. (eds) Petroleum Geology of North West Europe, Graham & Trotman, London, 611622. ALBRIGHT, W. A., TURNER,W. L. & WILLIAMSON,K. R. 1980. Ninian Field, U.K. Sector, North Sea. American Association of Petroleum Geologists, Memoir 30, 173 193. ALLEN, J. R. L. & K.gYE, P. 1973. Sedimentary facies of the Forest Marble (Bathonian), Shipton-on-Cherwell Quarry, Oxfordshire. Geological Magazine, ll0, 153-163. ANDREWS, I. J. & BROWN, S. 1987. Stratigraphic evolution of the Jurassic, Moray Firth. In: BROOKS,J. & GLENNIE,K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 785-795. ASHTON, M. 1980. The stratigraphy of the Lincolnshire Limestone Formation (Bajocian) in Lincolnshire and Rutland (Leicestershire). Proceedings of the Geologists' Association, 91, 203-223. BARNARD,P. C. & COOPER, B. S. 1981. Oil and source rocks of the North Sea area. In: ILLING,L. V. & HOBSON,G. D. (eds) Petroleum Geology of the Continental Shelf of North- West Europe. Heyden, London. BARR, K. W., COLTER,V. S. & YOUNG, R. 1981. The geology of the Cardigan BaySt. George's Channel Basin. In: ILLtNG~L. V. & HOBSON,G. D. (eds) Petroleum Geology of the Continental Shelf of North- West Europe. Heyden, London, 432443. BOOTE, D. R. D. & GUSTAV,S. H. 1987. Evolving depositional systems within an active rift, Witch Ground Graben, North Sea. In: ILL1N6, U V. & HOBSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North- West Europe. Heyden, London, 819-833. BUTT, M. H. P. 1978. The origin and development of the continental margins between the British Isles and southeastern Greenland. In: BowEs, D. R. & LEAKE, B. E. (eds) Crustal evolution in Northwestern Britain and adjacent areas. Geological Journal Special Issue, 10, 377-392. BRAOSlaAW,M. J. 1978. A facies analysis of the Bathonian of eastern England. DPhil thesis, University of Oxford. -& BATE, R. H. 1982. Lincolnshire borehole proves greater extent of the Scarborough Formation (Jurassic: Bajocian). Journal of Micropalaeontology, 1, 141-147. -& PENNEY, S. R. 1982. A cored Jurassic sequence from north Lincolnshire, England: stratigraphy, facies analysis and regional context. Geological Magazine, 119, 113-228. BREKKE,T., PEGRUM,R. M. & WATTS,P. B. 1981. First exploration results in Block 31/2 (Offshore Norway). In: Norwegian Symposium on Exploration ( NSE-81), Bergen. Norwegian Petroleum Society, Paper NSE/16. BROOKS,J. & GLENNIE,K. W. (eds) 1987. Petroleum Geology of North-West Europe. Graham & Trotman, London. BROWN, S. 1986. Jurassic. In: GLENNIE, K. W. (ed.) Introduction to the Petroleum Geology of the North Sea. Blackwell, Oxford, 133-159. - - , RICHARDS,P. C. & THOMPSON,A. R. 1987. Patterns in the deposition of the Brent Group (Midle Jurassic) UK North Sea. In: BROOKS,J. & GLENNIE,K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 899 913. BUDDING, M. C. d~. INGL1N,H. F. 1981. A reservoir geological model of the Brent Sands in Southern Cormorant. In: ILLtNG, L. V. & HUDSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 326--334. CASTON,V. N. D., DEARNLEY,R., HARRISON,R. K., RUNDLE,C. C. & STYLES,M. T. 1981~ Olivine-dolerite intrusions in the Fastnet Basin. Journal of the Geological Society, London, 138, 31-46. CHOWDStmV, A. N. 1982. Smectite, zeolite, biotite and apatite in the Corallian (Oxfordian) sediments of the Baulking area in Berkshire, England. Geological Magazine, 119, 487-496. COOK, D. R. 1987. The Goban Spur--Exploration in a deep water frontier basin. In: BROOKS, J. & GLENNIE,K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 623-632. COPE, J. C. W. 1984. The Mesozoic history of Wales. Proceedings of the Geologists' Association, 95, 373-385. -1985. The terminal Jurassic Stage. In; Proceedings of the International Symposium on Jurassic Stratigraphy, Erlangen, September 1-8. 1984. Geological Survey of Denmark, vol. III, 446-456. 9 GETTY,T. A., HOWARTH, M. K. & TORRENS, H. S. 1980a. d correlation of Jurassic" rocks in the British Isles. Part One. Introduction and Lower Jurassic'. Geologicat Society, London, Special Report, 14. , DUFF, K. L., PARSONS,C. F., TORRENS,H. S., WIMBLEDON,W. A. & WRIGHT, J. K. 1980b. A correlation of Jurassic rocks in the British Isles. Part Two: Middle and Upper Jurassic. Geological Society London, Special Report, 15. CORmN, S. G. 1980. A .facies analysis of the Lower-Middle Jurassic boundary beds o] northwest Europe. PhD thesis, University of Birmingham. Cox, B. M., LOTT, G. K., THOMAS,J. E. & Wn_KINSON, I. P. 1987. Upper Jurassic stratigraphy of four shallow cored boreholes in the U.K. sector of the southern North Sea. Proceedings of the Yorkshire Geological Society, 46. 97-109. DIXON,J. E., FITTON,J. G. & FROST,R. T. C. 1981. The tectonic significance of postCarboniferous igneous activity in the North Sea Basin. In: ILLING, L. V. & HOBSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 121 137. Dogf:, A. G., VOLLSET, J. V. & HAMAR,G. P. 1985. Correlation of the offshore sequences referred to the Kimmeridge Clay Formation--relevance to the Norwegian sector. In: THOMAS, B. M. et al. (eds) Petroleum Geochemistry m Exploration of the Norwegian S/will Graham & Trotman, London, 27-37.
DUNNING, F. W. (ed.) 1985. Geological structure of Great Britain. Ireland and surrounding shelf seas. Geological Society, London, Mapchart. EVANS, C. D. R., LOTI, G. K. & WARRINGTON,G. 1981. The Zephyr (1977) wells, South-Western Approaches and Western English Channel. Institute of Geological Sciences, Report 8118. - - , CHESHER,J. A., DEEGAN,C. E. & FANNIN,N. G. T. 1982. The offshore geology of Scotland in relation to the IGS Shallow Drilling Programme, 1970-1978. Institute of Geological Sciences, Report 81/12. EYNON, G. 1981. Basin development and sedimentation in the Middle Jurassic of the northern North Sea. In: ILLING,L. V. & HOB.SON,G. O. (eds) Petroleum Geology of the Continental Shelf of North-West Europe, Heyden. London, 196-204. FENTON, J. P. G. 1980. Palynological investigation of the Middle Toarcian to Lower Callo vian strata of Lincolnshire and Northamptonshire. PhD thesis, University of Sheffield. - - & FISHER, M. J. 1978. Regional distribution of marine microplankton in the Bajocian and Bathonian of northwest Europe. Palinologia Special Issue, 1, 233243. FRANCIS, J. E. 1983. The dominant conifer of the Jurassic Purbeck Formation, England. Palaeomology, 26, 277-294. FURNES,H., ELVSnORG,A. & MALM,O. A. 1982. Lower and Middle Jurassic alkaline magmatism in the Egersund Sub-basin, North Sea. Marine Geology, 46, 53-69. FYEE, J. A., ABBOTXS, I. & CROSnY, A. 1981. The subcrop of the mid-Mesozoic unconformity in the UK area. In: ILLING, U V. & HOaSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 236-244. GARDINER,P. R. R. & SHERIDAN,D. J. R. 1981. Tectonic framework of the Celtic Sea and adjacent areas with special reference to the location of the Varisean Front. Journal of Structural Geology, 3, 317-33 I. GEORGE, T. N. 1960. The stratigraphical evolution of the Midland Valley. Transactions of the Geological Society of Glasgow, 24, 32-107. GRAUE,E., HELLAND-HANSEN,W., JOHNSON,J., LOMO,L., NOTTVEDT,A., RUNNING, K., RYSETH,A. & STEEL, R. 1987. Advance and retreat of Brent Delta System, Norwegian North Sea. In: BROOKS, J. & GLENNIE, K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 915-937. HALLAM, A. 1975. Jurassic environments. Cambridge University Press, Cambridge. -1978. Eustatic cycles in the Jurassic. Palaeogeography, Palaeoclimatology, Palaeoecology 23, 1-32. HALLET.D. 1981. Refinement of the geological model of the Thistle Field. In: ILLING, L. V. & HOBSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 315-325. HAMAR,G. P., FJAERAN,T. & HESIEDAL,A. 1983. Jurassic stratigraphy and tectonics of the south-southeastern Norwegian offshore. Geologie en Mijnbouw, 62, 103114. HANCOCK, N. J. & FISHER, M. J. 1981. Middle Jurassic North Sea deltas with particular reference to Yorkshire. In: ILLING, L. V. & HOBSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 186-195. HAQ, B. U., HARDENBOL,J. & VAIL, P. R. 1987. Chronology of Fluctuating Sea Levels Since the Triassic. Science, 235, 1156--1166. HARKER, S. D., GUSTAV, S. H. d~. RILEY, L. A. 1987. Triassic to Cenomanian stratigraphy of the Witch Ground Graben. In: BROOKS,J. & GLENNIE, K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 809-818. HARRIS, J. P. 1984. Environments of deposition of Middle Jurassic sandstones in the Great Estuarine Group, N.W. Scotland. PhD thesis, University of Leicester. - & FOWLER, R. M. 1987. Enhanced prospectivity of the Mid-Late Jurassic sediments of the South Viking Graben, northern North Sea. In: BROOKS,J. & GLENNIE, K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 879-898. HELLEM, T., KJEMPERUD,A. & OVREBO,O. K. 1986. The Troll Field: a geological/ geophysical model established by t h e PL085 Group. In: SPENCER, A. M. et al. (eds) Habitat of hydrocarbons on the Norwegian Continental Shelf. Graham & Trotman, London, 217-238. HERNGREEN, G. F. W. & DE BOER, K. F. 1985. Palynology of the "Upper Jurassic' Central Graben, Scruff and Delfland groups in the Dutch part of the North Sea Continental Shelf. In. Proceedings of the International Symposium on Jurassic Stratigraphy, Erlangen, September 1-8, 1984. Geological Survey of Denmark, vol. III, 695-713. - & WONG, Th. E 1989. Revison of 'Late Jurassic' stratigraphy of the Dutch Central North Sea Graben. Geologie en Mijnbouw, 68, 73-105. HITCHIN, K. & RITCHIE,J. D. 1987. Geological review of the West Shetland area. In: BROOKS, J. & GLENNIE,K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 737-749. HOORN, B. VAN, 1987. Structural evolution, timing and tectonic style of the Sole Pit inversion. Tectonophysics, 137, 239-284. HOUSE, M. R. 1985. A new approach to an absolute timescale from measurements of orbital cycles and sedimentary microrhythms. Nature, 315, 721-725. HOwAat), A. S. 1985. Lithostratigraphy of the Staithes Sandstone and Cleveland Ironstone formations (Lower Jurassic) of north-east Yorkshire. Proceedings of the Yorkshire Geological Society, 45, 261-275. HOWITT, F., ASTON,E. R. & JAcQuL M. 1975. The occurrence of Jurassic volcanics in the North Sea. In: WOODLAND,A. W. fed.) Petroleum and the Continental Shelf of North West Europe. Volume 1: Geology. Applied Science, Barking, 379387. HUDSON,J. D. 1964. The petrology of the sandstones of the Great Estuarine Series, and the Jurassic palaeogeography of Scotland. Proceedings of the Geologists' Association, 75, 499-527. - 1980. Aspects of brackish-water facies and faunas from the Jurassic of northwest Scotland. Proceedings of the Geologists' Association, London, 91, 99-105. - - & HARRIS,J. P. 1979. Sedimentology of the Great Estuarine Group (Middle Jurassic) of North-West Scotland. S.l'mposium Sedimentation Jurassique W. Europken, Paris. 9 I0 May, 1977. Association Sedimentologie Franeais, Special Publication I, 1-13. ILLING, L. V. & HOBSON, G. D. (eds) 1981. Petroleum Geolog.l" of the Continental Shelf of North- West Europe. Heyden, London. JACKSON. D. I., MULHOLLAND. P., JONES. S. M. & WARRINGXON,G. 1987. The Geological framework of the East Irish Basin. In: BROOKS,J. & GLENNIE.K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman. London, 191 203. JENSEN, T. F., HOLT,i, L., FRANDSEN, N. & MICHELSEr~,O. 1986. Jurassic-Lower Cretaceous lithostratigraphic nomenclature for the Danish Central Trough. Danmarks Geologiske Undersogelse. Serie A. 12. JOHNS. C. R. & ANDREWS, I. J. 1985. The petroleum geology of the Unst Basin, North Sea. Marine and Petroleum Geology, 2. 361-372.
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JOHNSON, H. D., MACKAY,T. A. & STEWART,D. J. 1986. The Fulmar Oil-field
SKARPNES,O,, HAMAR,G. P., JAKOBSSON,K. H. & ORMAASEN,D. E. 1980. Regional
(Central North Sea): geological aspects of its discovery, appraisal and development. Marine and Petroleum Geology, 3, 99-125. KARLSSON,W. 1986. The Snorre, Statfjord and Gullfaks oilfields and the habitat of hydrocarbons on the Tampen Spur, offshore Norway. In: SPENCER,A. M. et al. (eds) Habitat of hydrocarbons on the Norwegian Continental Shelf. Graham & Trotman, London, 181-197. KENT, P. E. 1975. The tectonic development of Great Britain and the surrounding seas. In: WOODLAND,A. W. (ed.) Petroleum and the Continental Shelf of North West Europe. Volume 1: Geology. Applied Science, Barking, 3-28. - 1980a. Subsidence and uplift in East Yorkshire and Lincolnshire: a double inversion. Proceedings of the Yorkshire Geological Society, 42, 505-524. -1980b. Eastern England from the Tees to the Wash (British Regional Geology) HMSO. KIRK, R. H. 1980. Statfjord Field--a North Sea Giant. Ninian Field, U.K. Sector, North Sea. American Association of Petroleum Geologists, Memoir 30, 95-116. KNOX, R. W. O'B. 1977. Upper Jurassic pyroclastic rocks in Skye, west Scotland. Nature, London, 265, 323-324. KOCH, J.-O. 1983. Sedimentology of Middle and Upper Jurassic sandstone reservoirs of Denmark. Geologie en Mijnbouw, 62, 93-102. LARSEN,R. M. & JARVm, L. J. 1981. The Geology of the Sleipner Field Complex. In: Norwegian Symposium on Exploration ( NSE-81), Bergen. Norwegian Petroleum Society, Paper NSE/15. LINSLEY, P. N., POTTER,H. C., MCNAB, G. & RACHER, D. 1980. The Beatrice Field, Inner Moray Firth, U.K. North Sea. Ninian Field, U.K. Sector, North Sea. American Association of Petroleum Geologists, Memoir 30, 117-129. LIVERA,S. E. & LEEDER,M. R. 1981. The Middle Jurassic Ravenscar Group ('Deltaic Series') of Yorkshire. Proceedings of the Geologists'Association, 92, 241-250. MEGNIEN, C. & MI~GNIEN,F. (eds) 1980. Synth6se g~ologique du bassin de Paris. Mbmoires du Bureau de Recherches Gkologiques et Minidres, 101-103. MtLLSON, J. 1987. The Jurassic evolution of the Celtic Sea basins. In: ILLING,L. V. & HOBSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 599-610. MORRIS, K. A. 1980. Comparison of major sequences of organic-rich mud deposition in the British Jurassic. Journal of the Geological Society, London, 137, 157-170. MORTON, A. C. & HUMPHREYS, B. 1983. The petrology of the Middle Jurassic sandstones from the Murchison Field North Sea. Journal of Petroleum Geology, 5, 245-260. MORTON,N. 1983. PalaeoculTents and Palaeo-environmentsof part of the Bearreraig Sandstone (Middle Jurassic) of Skye and Raasay, Inner Hebrides. Scottish Journal of Geology, 19, 87-95. NAM, 1980. Stratigraphic nomenclature of the Netherlands. Nederlandse Aardolie Maatschappij B.V. and Rijks Geologische Dienst. NAYLOR, D. & SHANNON,P. M. 1982. Geology of Offshore lrelandand West Britain. Graham & Trotman, London. OSCHMANN,W. 1988. Kimmeridge Clay sedimentation--a new cyclic model. Palaeogeography, Palaeoclimatology, Palaeoecology, 65, 217-251. PARRY, C . C . , WHI'rLEY, P. K. J. & S1MPSON, R. D. H. 1981. Integration of palynological and sedimentological methods in facies analysis of the Brent Formation. In: ILLING,L. V. & HOaSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North- West Europe. Heyden, London, 205-215. PERROT,J. & VAN DER POEL,A. 1987. Zuidwal: A Neocomian gas field. In: BROOKS, J. & GLENNIE,K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 325-335. RICHARDS, P. C., BROWN, S., DEAN, J. M. & ANDERTON, R. 1988. A new palaeogeographic reconstruction for the Middle Jurassic of the northern North Sea. Journal of the Geological Society, London, 145, 883-886. ROBINSON,K. W., SHANNON,P. M. & YOUNG,D. G. G. 1981. The Fastnet Basin; an integrated analysis. In: ILLING,L. V. & HOBSON,G. D. (eds) Petroleum Geology of the Continental Shelf of North- West Europe. Heyden, London, ~4~ 454. SIMPSON, R. D. H. & WHITLEY, P. K. J. 1981. Geological input to reservoir simulation of the Brent Formation. In: ILLING,L. V. & HOBSON,G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 310-314.
Jurassic setting of the North Sea north of the Central Highs. In: The Sedimentation of the North Sea Reservoir Rocks, Geilo. Norwegian Petroleum Society, Paper 13. SYKES, R. M. 1975. Facies and faanal analysis of the Callovian and Oxfordian Stages (Middle-Upper Jurassic) in northern Scotland and east Greenland. DPhil thesis, University of Oxford. TAN, F. C. & HUDSON, J. D. 1971. Carbon and oxygen isotopic relationships of dolomites and coexisting calcites, Great Estuarine Series (Jurassic), Scotland. Geochimica Cosmochimica Acta, 35, 755-767. THOMAS,J. B. 1975. The geology of the southern North Sea. In: Offshore Europe '75. Offshore Services Magazine, Paper OE-75 213. THOMAS, B. M., MOLLER-PEDERDEN, P., WHITAKER,M. F. & SHAW, N. D. 1985. Organic facies and hydrocarbon distributions in the Norwegian North Sea. In: THOMAS, B. M. et al. (eds) Petroleum Geochemistry in Exploration of the Norwegian Shelf. Graham & Trotman, London, 3-26. TURNER,C. C., COHEN,J. M., CONNELL,E. R. & COOPER,D. M. 1987. A depositional model for the South Brae Oilfield. In: BROOKS, J. & GLENNIE, K. W. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 853-864. TYSON, R. V., WILSON, R. C. L. & DOWNIE, C. 1979. A stratified water column environmental model for the type Kimmeridge Clay. Nature, 377-380. VAIL, P. R., MITCHUM, R. M., TODD, R. G., WIDMIE~:, J. M., THOMPSON, S., SANGREE,G. n., BUBB,J. N. ,g" HATLELID,W. G. 1977. Seismic Stratigraphy and Global Changes of Sea Level. American Association of Petroleum Geologists, Memoir 26, 49-212. - & TODD, R. G. 1981. Northern North Sea Jurassic unconformities, chronostratigraphy and sea-level changes from seismic stratigraphy. In: ILLING,L . V . & HOasoN, G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 216-235. VOLLSET, J. & DORL A. G. 1984. A revised Triassic and Jurassic lithostratigraphic nomenclature for the Norwegian North Sea. NPD Bulletin No. 3. WALSH,P. T., BOULTER,M. C., IJTABA,M. & URBANI,D. M. 1972. The preservation of the Neogene Brassington Formation of the southern Pennines and its bearing on the evolution of Upland Britain. Journal of the Geological Society, London, 128, 519-559. WARRINGTON, G. & OWENS, B. 1977. Micropalaeontological biostratigraphy of offshore samples from southwest Britain. Institute of Geological Sciences, Report 77/7. WHITTAKER,A. (ed.) t985. Atlas of onshore sedimentary basins in England and Wales. Blackie, Glasgow. WUHE, D. H. VAN 1987. Structural evolution of inverted basins in the Dutch offshore. Tectonophysics, 137, 171-219. WIMBLEDON,W. A. 1985. The Portlandian, the terminal Jurassic stage in the Boreal Realm. In: Proceedings of the International Symposium on Jurassic Stratigraphy, Erlangen. September 1--8, 1984. Geological Survey of Denmark, vol. III, 534549. - 1987. Rhythmic sedimentation in the Late Jurassic-Early Cretaceous. Proceedings of the Dorset Natural History and Archaeological Society, 108, 127-133. - - & HUNT, C. O. 1983. The Portland-Purbeck junction (Portlandian-Berriasian) in the Weald, and correlation of latest Jurassic--carly Cretaceous rocks in southern England. Geological Magazine, 120, 267-280. WOODHALL, D. & KNOX, R. W. O'B. 1979. Mesozoic volcanism in the northern North Sea and adjacent areas. Bulletin of the Geological Survey of Great Britain, 70, 34-76. ZIEGLER,P. A. 1981. Evolution of sedimentary basins in North-West Europe. In: ILLING,L. V. & HOnsoN, G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 3-39. 1982. Geological atlas of western and central Europe. Shell International Petroleum Maatschappij B. V., 2 vols. -1988. Evolution of the Arctic-North Atlantic and the Western Tethys. American Association of Petroleum Geologists, Memoir 43. n o t e . The Middle Jurassic maps were amongst the first submittted. They were partially updated, and the area of J5 and J8 expanded by PFR.
Editors'
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Cretaceous J. M. HANCOCK
EARLY
CRETACEOUS
The Cretaceous Period lasted for about 70 million years. During this time there was a major change in the sedimentary history of the area as tectonism died down and deposition started of an extensive blanket of coccolith ooze: the Chalk. The change took place mainly over a brief interval across the Albian/Cenomanian (Lower/Upper Cretaceous) boundary, at about 95 Ma. Until thattime crustal extension along the Arctic-North Atlantic megarifts continued to influence the tectonic evolution of northwest Europe (Ziegler 1982, 1988). This tensional rrgime caused rifting and block faulting, particularly across the Jurassic-Cretaceous boundary (Late Cimmerian movements) and in the mid Aptian (Austrian phase). During the latter phase, sea-floor spreading commenced in the Biscay and central Rockall Rifts. The northern part of the Rockalt Rift began to widen too, possibly by crustal stretching rather than sea-floor spreading (Ziegler 1988, p. 75). During the Albian the regional pattern began to change and by the beginning of the Cenomanian rifting had effectively ceased away from the Rockall/Faeroe area. Most of the Jurassic sedimentary basins continued as depositional areas during the Early Cretaceous, but the more extensive preservation of Lower Cretaceous sediments provides firmer constraints on some of the geographical reconstructions. The marked sea-level fall across the Jurassic-Cretaceous boundary isolated the more southerly basins as areas of non-marine sedimentation, and it was not until the beginning of the Aptian that they became substantially marine. The extent of emergence of highs in the North Sea area is difficult to assess, especially where no sediments are preserved. In particular, Fyfe et aL (1981) indicate that the Shetland Platform may have been largely emergent during the Lower Cretaceous while Day et al. (1981) suggested that sediments extended over much of the area but were subsequently largely eroded. Much of the Fladen Ground Spur and Halibut Horst may have remained emergent throughout the Lower Cretaceous, when they were a source of sand at times (Boote & Gustav 1987, fig. 13). The Tampen Spur was uplifted and eroded during the late Cimmerian movements, then partially draped by Hauterivian/Barremian limestones and marls (Karlssan 1986). Further south, The Vestland Arch forms the eastern flank of the north Central and Viking Grabens, and comprises a series of eastward rotated fault blocks bounded to the southwest and west by large normal faults (Skjerven et al. 1983). The main component highs, from north to south, are the Bergen, Utsira, Jaeren, Hidra and Mandal highs. There are too few data available to assess the individual history of each high accurately, but sediments extend over the arch in places. However, facies distributions indicate that the Jaeren High may have remained emergent till the beginning of the Albian (Hesjedal & Hamar 1983). It is equally difficult to assess how much of the Mid North Sea High (essentially lying between 55 ~ and 56 ~ North) was submerged. A limited number of wells were drilled in the early stages of North Sea exploration but the released data do not detail the age of the Lower Cretaceous mudrocks, which Day et al. (1981, fig. 3) show to extend over the whole of the high. Fyfe et al. (1981, fig. 3) indicate in their 'Cretaceous lapout map' that over most of the area between 540 and 56~ and west of 3~ it is Aptian/Albian strata that overly pre-Cretaceous beds. However, within that area sediments as old as late Ryazanian have been proved in places (Map K I ) so it may be that much of the high had never been emergent across the JurassicCretaceous boundary. Much of central England is portrayed as land through early Cretaceous time, though radically different geographies have been postulated which would flood extensive areas north of the Wessex Basin and Anglo-Brabant landmass (Kaye 1966; Allen 1981). Facies evidence supports the more conservative reconstructions. However, during the Aptian-Albian sea level rises, shorelines gradually retreated westward, while the Anglo-Brabant landmass was progressively submerged to be completely flooded by the Cenomanian. Once again geographical reconstruction becomes increasingly speculative towards the northwestern part of the map. A released well to the northwest of Ireland (12/13-1A) shows that the Donegal Basin was accumulating sediments, predominantly non-marine at first but becoming marine by the Barremian (Ziegler 1988, plate 27). To the northwest of Scotland sediments occur in two areas. In the Solan Basin calcareous mudstones accumulated throughout the early Cretaceous (Meadows et al. 1987). The West Shetland Basin is a half-graben widening towards the structurally deeper southwestern end. It was bounded to the west by the partially emergent Rona Ridge and to the east by the Shetland Spine Fault. Throughout the early Cretaceous sandstones and mudstones (with thin limestones and coals) accumulated, reaching over 1100 m in thickness. The sands were deposited 9 1992The GeologicalSociety
& P. F. R A W S O N
as coalescing submarine fans and talus slopes, shed by movement along the Spine Fault, and shaling out westwards (Hitchen & Ritchie 1987; Meadows et al. 1987). Albian elastics were shed from the Rona Ridge to the adjacent Faeroe Rift following pre-Albian subsidence there (Map K3). Vulcanicity occurred intermittently throughout the Lower Cretaceous, indicated principally by thin bentonitic horizons in clay sequences; important occurrences are discussed under the appropriate map. Aptian/Albian lavas have been proved in the outer Western Approaches Trough (Map 3). As with the Jurassic, biostratigraphical control in onshore marine sequences can usually be refined to an ammonite zone (see Rawson et al. 1978). Dating of non-marine and offshore marine sequences is more generalized. PFR
KI: Late Ryazanian (Berriasian) During the Ryazanian sea levels began to rise following the Iowstand across the Jurassic Cretaceous boundary. Much of the area was still emergent but the Biscay Rift, much of the North Sea region, the West Shetland Basin and presumably the Rockall-Faeroe and East Greenland rifts were marine. There were also important areas of non-marine sedimentations in the Celtic Sea, Western Approaches, Channel, Weald and Paris Basins. In the North Sea area an early late Ryazanian ( s t e n o m p h a l u s Zone) rise in sea level flushed out the previously stagnant bottom waters (Rawson & Riley 1982), causing a major facies change from 'hot' shales (Kimmeridge Clay and correlative formations) to the calcareous mudrocks of the Vaihall and Speeton Clay Formations (Cromer Knoll Group). The Valhall Formation is widespread north of the mid-North Sea-Ringkobing Fyn highs and extends into the Danish Central Graben. Its distribution is based mainly on Day et al. (1981, fig. 3) and Hesjedal & Hamar (1983, fig. 2b). In the deeper water areas the lowest beds are undoubtedly of late Ryazanian age, but towards the highs they are sometimes younger. Over starved highs the Cromer Knoll Group may be represented by condensed limestones--the Utvik Formation (formerly 'Barremian limestone')--which sometimes includes a late Ryazanian component. At the northeastern extremity of the Mid North Sea High the Auk Ridge was emergent and was the source of the Devil's Hole Formation sands (type well 29/25-1). In the Danish Central Graben and in Norwegian type well 2/11-1 the lowest part of the Valhall Formation is more calcareous than usual, consisting of interbedded claystones, marlstones and limestones of the Leek Member (Jensen et al. 1986). It occurs in basinal areas as well as on highs, and is not therefore comparable with the condensed Utvik Formation. In the vicinity of Danish well V-I the locally developed Vyl Formation (43 m in the type well) consists of siltstones with interbeds of fine-grained sandstone, silty claystone and marlstone. It probably represents a gravity flow deposit from active fault scarps to the east (Jensen et al. 1986). Similar beds may occur elsewhere along the western margin of the Ringkobing Fyn High. For reasons outlined above, most of the Mid North Sea High is believed to have been submerged. The facies distribution is based mainly on Fyfe et al. (1981, fig. 3) and released wells. To the south, marine sedimentation took place mainly in the British sector, extending onshore to eastern England. On the East Midlands Shelf shallow marine sands and silty sands of the Spiisby Sandstone Formation accumulated; these pass laterally to mudrocks of the Speeton Clay Formation in the vicinity of the Dowsing Fault line (Glennie & Boegner 1981, fig. 3c). The Speeton Clay Formation is best known from its type locality on the Yorkshire coast, where a l b i d u m Zone ammonites occur. Offshore it extends northwards towards the southern margin of the Mid North Sea High, where late Ryazanian strata are proved in BGS borehole 81/43 and nearby BP well 42/13-1 (Lott et al. 1986). It also extends southeastwards towards the Sole Pit Trough but has been removed from the main part of the trough following later inversion. Further east, in the area of quadrant 49, sedimentation was strongly influenced by pre-Cretaceous highs, which the Lower Cretaceous sediments onlap (Crittenden 1982); on the map the whole of this area is shown as marine, but it is possible that there were some islands here as the basal sequence is missing on some highs. Late Ryazanian clays may be represented in 49/25-I and some adjacent wells by the lowest part of the Vlieland Shale Member. In the Dutch sector sediments are preserved in the Central Graben and the Vlieland, Broad Fourteens, Central Netherlands and West Netherlands basins (van Wijhe 1987; NAM 1980; Heybroek 1975, fig. 5). Over most of this area the fluviatile-deltaic sediments of the Delfland Formation continued to accumulate during the Ryazanian, prograding to the northwest. 131
132
CRETACEOUS
The sediments are mostly sands and mudrocks with some coals. However, the Dutch Central Graben was mainly marine and here the 'unconformity' (with supposed Ryazanian hiatus) between the Clay Deep (formerly Kimmeridge Clay pars) and Vlieland Formations marks a facies change (Herngreen & Wong 1989) that may represent the albidum Zone sea level rise. A transgression of that age brought brackish-water, eventually marine, Vlieland sands into the Vlieland Basin (Herngreen et al. 1989). By the beginning of the Valanginian the remaining non-marine basins had been flooded (NAM 1980). South of the Anglo-Brabant landmass increased tectonic activity accompanied by climatic change led to increased run-off from bordering highs into the Anglo-Paris Basin, Western Approaches Trough and Celtic Sea basins. Thus sands and mud (Hastings facies) spread over extensive areas to replace the Purbeck limestones and marls. The depositional environment was essentially one of deltas and braided streams flowing into very shallow, ephemeral lakes. The Celtic Sea basins probably drained eastward through the Bristol Channel Basin to join another drainage system from the Wessex Basin before flowing towards the East Midlands Shelf. Sea water apparently spilled southwestward at times from the shelf into these non-marine basins. The Western Approaches Trough probably drained towards the west as there is evidence of increasing marine influence in released wells in the Brittany Trough, leaving an emergent ridge from Conubia to Armorica, approximately along the Start-Cotentin line. Distribution of facies is generalized from Allen (1981), Ainsworth et al. (1987), Mrgnien & Mrgnien (1980), numerous boreholes in the Wessex Basin and released wells in the Western Approaches and Celtic Sea areas. Although the Irish Sea area was apparently emergent, the St George's Channel Basin may have been depositional as it contains Lower Purbeck sediments beneath the Tertiary unconformity. Further north, non-marine, predominantly arenaceous rocks occur in the Donegal Basin. Sedimentation was apparently occurring in the Solan and West Shetland Basins (see above). Volcanic activity in the albidum Zone is precisely dated at Speeton, where bentonitic bands occur in ammonite-bearing clays. The same horizons have also been identified in BGS offshore borehole 81/43, some 80 km to the ENE (Lott et al. 1986). The source of these may lie in an as yet undiscovered vent in the southern North Sea. Other possibly contemporaneous igneous activity is marked in the West Netherlands Basin by intrusive nephelinites and basanites (dated to 132 + 3 Ma but possibly older) and on the Cornubian Platform by the Wolf Rock phonolite, dated to c. 130Ma (Dixon et al. 1981). PFR
K2a: Mid Hauterivian From late Ryazanian to mid Hauterivian times there were several pulses of rising sea level, each followed by slight fall (Rawson & Riley 1982). There was no fundamental change in the distribution of land and sea but shorelines were gradually retreating. A mid Hauterivian (inversum Zone) rise led to the highest sea level in pre-Aptian times and is the interval chosen for Map K2a. Clay facies became particularly widespread, in both marine and non-marine areas. Over the North Sea the pattern was very similar to that of late Ryazanian times and the distribution is based mainly on references quoted for that interval. North of the Mid North Sea High calcareous mudrocks of the Valhall Formation were widespread while condensed limestones (Utvik Formation) continued to accumulate over some of the starved highs, though on others it was the mid Hauterivian sea-level rise that caused a change in facies from limestone to mudrock. Sediments are preserved as far south as BGS bores 74/12 (Evans et al. 1982) and 81/40 (Lott et al. 1985). The Mid North Sea High was apparently submerged though sediments are only known locally, especially on the southern margin in BGS borehole 81/ 43 and BP well 42/13-1 (Lott et al. 1986). Here and to the south the facies is similar to that of the Valhall Formation but the mudrocks are assigned to the Speeton Clay Formation as they are contiguous with the type outcrop in North Yorkshire. The mid Hauterivian interval also saw the maximum spread of mudrocks over the East Midlands Shelf (Tealby Clay Member), where silts became limited to the southern margin in North Norfolk (Dersingham Formation). At this sea-level peak there was probably a brief marine spillover from the shelf to the Weald Basin to form the mid-Weald Clay quasi-marine band (Rawson 1991). To the south of the Welsh and Anglo-Brabant landmasses non-marine sediments continued to accumulate. By the begining of the Hauterivian tectonic activity in the surrounding highs was apparently waning, sediment input was decreasing and sands gave way to more argillaceous sediments (Weald Clay Formation and equivalents). These accumulated predominantly in very shallow, constantly shifting lakes occupying the Wessex, Channel, Paris and Celtic Sea basins and probably the Western Approaches too. The Wessex and Celtic Sea lakes continued to drain towards the East Midlands Shelf, while drainage from the Western Approaches was still to the adjacent Biscay Rift, the Start-Cotentin Ridge forming a watershed. Several rivers continued to flow into the Wessex/Channel basins (Allen 1981) bringing detritus from the Anglo-Brabant, Armorican and Cornubian massifs. Distribution of facies is generalized from Allen (1981), Ainsworth et al. (1987), M~gnien & M~gnien (1980), numerous boreholes in the Wessex Basin and released offshore wells. To the northwest of the Irish and Scottish landmasses few sediments are preserved. The Donegal Basins was still non-marine, while marine sediments continued to accumulate in the Solan and West Shetland basins.
Although there is no evidence of mid Hauterivian vulcanicity, it should be noted that early and late Hauterivian bentonites have been recorded at Speeton and in BGS offshore borehole 81/43 (Lott et al. 1986). PFR
K2b: Late Aptian Sea levels fell slightly during latest Hauterivian to Barremian times, but a significant rise at the beginning of the Aptian brought marine conditions back to southern Britain for the first time since latest Jurassic times and created a temporary link with the East Midlands Shelf through the 'Bedfordshire Straits'. The link was apparently severed during the Austrian tectonic phase, which peaked in mid Aptian times when local tectonic uplift occurred and sea levels fell across Europe. They began to rise again in the late Aptian when a transgression across southern Britain in the nutfieldiensis Zone reestablished the connection with the East Midlands Shelf: that is the time interval chosen for the map. Mudrocks were still being deposited over extensive areas of the North Sea region; their distribution is modified from Hesjedal & Hamar (1983, fig. 5a) and Day et al. (1981, fig. 3). Over the central North Sea the mudrocks were assigned to the Valhall Formation by Deegan & Scull (1977). However, in much of the area, especially in the Norwegian and Danish sectors and the Witch Ground Graben, the Valhall Formation is now used in a more restricted sense (Fig. 1) and the late Aptian sediments are placed in the Sola Formation (Hesjedal & Hamar 1983; Jensen et aL 1986; Harker et al. 1987). This represents a facies change following the Austrian movements, from calcareous mudrocks of typical Valhail facies to non-calcareous, darker mudrocks with a higher organic carbon content. By the late Aptian much of the Vestland Arch was apparently submerged but crests of some component highs could have still formed small islands, particularly the Jaeren High (Hesjedal & Hamar 1983, figs 2, 3, 5). Sands derived from the latter accumulated along its southwest margin to form the Kopervik Formation. Fyfe et al. (1981, fig. 3) showed Aptian-Albian sediments extending over much of the Mid North Sea High. To the south, an area of more marly mudrocks extends westwards from the margin of the Sole Pit inversion over the East Midlands Shelf and southern Cleveland Basin (Speeton). Offshore and also at Speeton the marls are assigned to the Speeton Clay Formation (Rhys 1974) while on the onshore part of the shelf they form the Sutterby Marl Member. To the east of the Sole Pit inversion Crittenden (1982) summarized several wells in block 49 (UK), where the Upper Aptian appears to be missing between the Lower Holland Marl (Lower Aptian) and the Middle Holland Shale (Lower Albian). The geography of the areas to the west and south of the Anglo-Brabant Landmass is difficult to reconstruct. In the tidally swept Bedfordshire Straits marine sands (Woburn Formation) and local sponge gravels (Faringdon Greensand) accumulated. The seaway was apparently narrow, with brachiopod faunas of the opposing shorelines preserved (Middlemiss 1961). To the south, marine Lower Greensand sediments occur in the Wessex and Channel basins and Western Approaches Trough, but none appear to be preserved in the adjacent part of the Paris Basin. There was probably a seaway through the latter area, but both that and the seaway through the Western Approaches Trough may be narrower than indicated on the map. The preserved sediments are scattered over three main areas. In the Weald Basin the Sandgate Formation is predominantly argillaceous in the east but becomes sandy (Bargate facies) to the west. An isolated remnant of the nutfieldiensis Zone transgression occurs at Seend (ferruginous sands). Equivalent sandy beds in the western Channel Basin form the Ferruginous Sands Formation of the Isle of Wight and adjacent areas. In the offshore part of the basin the Lower Greensand has been sampled but there is insufficient data for facies to be indicated on the map. In the Western Approaches Trough the late Aptian is poorly known but glauconitic sandstones with mudstone interbeds occur in three wells. A similar facies occurs in well LET 1 in the Brittany Basin. To the southwest of the Celtic Sea area, tectonic collapse of highs bordering the Rockall Rift allowed seawater to spread through the Fastnet and Celtic Sea basins during middle-late Barremian times. There is evidence of renewed uplift during the Aptian, but the late Aptian is represented by shallow marine calcareous claystones and siltstones (Ainsworth et aL 1987), with ostracod faunas showing a strong Tethyan influence. The Donegal Basin had been invaded by the sea by about the beginning of the Barremian, and late Aptian sediments in well 12/13-1A are predominantly argillaceous with some limestones and sand. Much further to the northeast, sedimentation continued in the Solan and West Shetland basins. The nutfieldiensis Zone was a period of widespread volcanic activity in northwest Europe. This is best documented in southern England where thick accumulations of secondary bentonite washed off nearby land areas accumulated as 'Fullers' Earth' in the Woburn and Sandgate Formations. Primary bentonites of approximately the same age are recorded in BGS borehole 81/40 on the southern margin of the Forth Approaches Basin (Lott et al. 1985), and the same event is also documented in north Germany (Gaida et al. 1978). Such vulcanicity continued into the Albian (Jeans et al. 1982). The source of the ashfalls may have been in the Dutch offshore sector, where several wells penetrated basaltic intrusives giving radiometric dates between 95 and 106Ma (Dixon et al. 1981). A major phase of basalt extrusion in the outer part of the Western Approaches Trough (just off the map) apparently started in the Aptian but extended into the Albian. There is clear evidence of subaerial weathering of
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individual flows. This acitivity is related to the initiation of sea-floor spreading in the nearby Bay of Biscay PFR
K3: Latest Albian Sea levels rose more rapidly during the Albian, though in the Western Approaches and Celtic Sea areas there was a brief regression during the Early Albian. There was still a regional tilt to the east but shorelines gradually migrated westward as sea levels rose. In the North Sea previously emergent portions of the Vestland Arch, Mid North Sea High, Ringk~bing-Fyn High and Dogger High were probably flooded by the dispar Zone, the time interval for Map K3. Late Albian sediments occur over most of the North Sea area; their distribution is based mainly on Day et al. (1981, fig. 3), Fyfe et al. (1981, fig. 3), Jensen et al. (1986) and Hesjedal & Hamar (1983, fig. 6). They consist mainly of reddish coloured calcareous mudrocks and marls, with impure limestones in places, and generally become more calcareous upward. However, in some areas, especially the Inner Moray Firth and Forth Approaches Basins, there is little well control to confirm the facies. In the Central North Sea these beds are placed in the Rodby Formation (Middle and Upper Albian). This is normally about 15-30m thick but increases to about 44 m in the Danish Central Graben (Jensen et al. 1986) and as much as 210 m in the area of Norwegian block 16/6 (Deegan & Scull 1977). In some areas the formation occurs in red chalk facies, as in the southern Forth Approaches Basin where BGS borehole 81/40 cored 7 m of 'red chalk' of mid Albian-early Cenomanian age (Lott et al. 1985). Limestones accumulated over the Jaeren High (Hesjedal & Hamar 1983, figs 2, 3, 5), while in the area of the Witch Ground Graben the formation becomes sandy (Harker et al. 1987). Mid-late Albian sands (Flora Formation) were deposited over the buried Sele High (Hesjedal & Hamar 1983, fig. 3a). Red coloured marls are widespread in the southern North Sea Basin. To the east of the Sole Pit Trough they are usually assigned to the Upper Holland Marl Member of the Holland Formation; again, the calcareous content tends to increase upward (Crittenden 1982). The sediments were eroded from t~e Sole Pit Trough during later inversion but to the west a more calcareous sequence appears, consisting of the impure limestones and marls of the 'Red Chalk' (= Hunstanton) Formation. Well 48/22-2 is type well for the offshore development, here 29m thick (Rhys 1974). Further west, in the onshore area, the Hunstanton Formation consists of red condensed limestones with subordinate marl, rarely more than 3 m thick. In north Norfolk the Hunstanton Formation passes laterally into calcareous clays, the Gault Clay Formation. This facies extends southward into an extensive development over the Wessex and Paris Basins. However, through the later Albian renewed uplift to the west caused glauconitic sands to prograde progressively further east over southern England, to form the Upper Greensand Formation: the dispar Zone probably represents the maximum extent of this facies. The formation is strongly transgressive towards Cornubia, overstepping earlier Mesozoic formations to rest on Devonian and Carboniferous rocks in south Devon. Similar sands occur also in parts of the Paris Basin (Mrgnien & Mrgnien 1980). There is very little sea floor outcrop in the Western Approaches Trough but DSDP hole 402A, sea floor samples, several released wells and BGS shallow borings (Lott et al. 1980) give reasonable coverage, especially adjacent to the Cornubian Massif where sandstones, sometimes glauconitic, are widespread. In the axial part of the trough the sediments are more argillaceous, becoming calcareous westward towards the continental margin. Argillaceous sands occur in the Brittany Trough. The Apto-Albian extrusion of basalts in the outer part of the Western Approaches had apparently ceased by the late Albian. To the west of the Cornubian Massif the BGS have discovered an extensive area of bioclastic limestone of late Albian to early Cenomanian age (Lott et al. 1980). Further into the Celtic Sea area there are two main areas where sediments are preserved. Published information for the Fastnet and North Celtic Sea Basins is limited and lithological changes through time are not well documented so that the facies portrayed on the map are generalized. The late Albian appears to be represented in the 'greensand' unit--sands and clays of predominantly marine aspect, possibly more calcareous in the Fastnet Basin (Ainsworth et al. 1987). Released wells in the northeastern part of the South Celtic Sea Basin suggest that mudrocks pass into sands towards the Pembroke Arch. Between the Celtic Sea and the area of the West Shetland Basin only a single record of Albian sediments is known--in the Donegal Basin where well 12/13-1A penetrated chalks. There is no direct evidence to indicate whether the Irish Sea area was still land. Sedimentation continued in the Solan and West Shetland Basins. Following major pre-Albian subsidence to the west of the Rona Ridge, submarine fans spilled into the Faeroe Rift during the Albian (Meadows et al. 1987). PFR LATE CRETACEOUS The overall rise in sea level during the Late Cretaceous meant that the seas were more widespread than at any other time during the Mesozoic, but the actual limits have always been a contentious subject. The Chalk is widespread offshore as well as onshore, but the very limited amounts of neritic non-chalk facies over all northwest Europe means that there is little evidence
for the position of any shoreline. There are substantial areas where there are no extant Upper Cretaceous sediments of any sort but which must have been submerged for at least part of the period. It is particularly difficult to decide how far west and north of the present outcrop the Chalk originally extended. The limits shown on Maps K4a and K4b are no more than intelligent guesswork, based on weak analogies. On the one hand there is the cogent argument of Cope (1984) that the speed of erosion and the amount of Chalk removed before the mid Eocene in southern England means that the whole Cretaceous cover could have been removed during the Tertiary in regions where there were no basalts to protect it. On the other hand there is the fact that in Europe generally it seems remarkably difficult to really remove all traces of a Cretaceous cover. At first thought, each occurrence of Upper Cretaceous material in outlying regions seems to be a special local accident (Hancock 1975). One thinks of Campanian chalk in collapse structures within the Namurian in County Kerry (Waish 1966), or the Campanian chalk in a volcanic neck in Arran (Peach et al. 1901). But bits and pieces of Upper Cretaceous are found on many massifs of northern Europe where there is no general Cretaceous cover. Nowhere in northern England or Wales has anyone yet recorded such outliers of Cretaceous, say within solution cavities in Palaeozoic limestones. Such cavities exist in the Carboniferous Limestone but no-one has even suggested that their infill is Cretaceous. Thus the 'pocket deposits' of Derbyshire are probably all Tertiary (Ford & King 1968). Therefore it may well be that the original extent of the Chalk was not far from the present outcrop. One is still left with the difficulty that the Late Cretaceous seas m u s t have extended some appreciable distance from the present outcrop of the relatively deep water chalk facies, yet no trace of a nearer shore more arenaceous facies has ever been found in Wales and north or northeast of Somerset in England. Part of the explanation could be that this is a region where there was no belt of neritic facies. For the reconstructions given here it has been assumed that the Campanian extended far beyond the Cenomanian, as is known to have happened in Ireland, Cornubia, the Ardennes and in Sweden. JMH
K4a: Early Cenomanian The Cenomanian stage is transgressive: at various localities in the Inner Hebrides one can see patches of Cenomanian (proved Lower Cenomanian in east Mull) resting on various stages of the Jurassic; and in southeast Antrim where Lower Cenomanian lies on Sinemurian. In eastern England this transgression is reflected by the upward passage into chalk facies low in the stage. There are considerable uncertainties on the distribution of facies. Along the English and French coasts we often have details for each subzone. In contrast, some of the offshore regions are little more than guesswork. When we have an exact knowledge of the stratigraphy, the data used are for the middle to upper part of the Lower Cenomanian, i.e. in places like the Isle of Wight or Folkestone, it is the chalk-marl facies of the saxbii and dixoni Zones; apart from the difficulty of recognizing the carcitanense Zone in many areas, a map of the very lowest Cenomanian would be atypical of the stage. A further difficulty is the distinction between chalk and chalk-marl. All the Cenomanian chalks contain greater or lesser amounts of clay. When does one start to indicate a marl component? Here it has been limited to relatively prominent quantities somewhere in the Lower Cenomanian (or Cenomanian in general, Plenus Marls excluded). The other major problem is where to indicate nearer-shore facies such as grizzle and greensand. Their occurrences on the western side of the British Isles from Scotland to Normandy indicate that there was normally a belt of neritic sediments between the land and the open sea chalks or chalk-marl (Kennedy 1970; Juignet 1980). This is not true of the western margin of the Ardennes as shown in the reconstructions of Robaszynski (1981). Whilst much imagination has had to be used in the construction of this map, particularly in some offshore areas, a neritic belt has been inserted only where there is some positive evidence for one. The long uncertain stretches are: off the west coast of Ireland; on the eastern side of Scotland; and across northern and central England, with some evidence for the last (from the lack of sandy chalk at the base of the Cenomanian in Yorkshire and Lincolnshire) that there was little in the way of neritic sediment. In the northern North Sea, there are examples of probable neritic facies on the flanks of the East Shetland Platform (as in 9/17-1A) and it is widely agreed that the Platform was not submerged until very late in the Cretaceous, probably late Campanian. On the eastern margin of the Platform there are wackestones with much bioclastic debris rather than typical chalk, e.g. 3/16-1 (Amiri-Garroussi 1988). In considerable areas of the Viking Graben the Cenomanian is absent. In many cases this can be explained as post-Cenomanian erosion over uptilted fault-blocks, but in other wells it seems that there was simply no sediment that represented Cenomanian time, and Turonian or even Coniacian rests directly on Albian. In the Graben between the East Shetland Platform and the Utsira High there are Cenomanian mudstones (Shetland Group), but in the far north the Cenomanian is again represented by chalk. For some of the Norwegian sector the facies are not indicated because the information available is inadequate to distinguish Cenomanian from higher stages. In the region east and northeast of the Moray Firth Basin there is published information from wells in the Dutch Bank Basin, Witch Ground Graben and South Halibut Basin, and on neighbouring highs such as the
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Halibut Horst. The presence or absence of Cenomanian does not follow a simple pattern of absence over horsts and occurrence in basins: the local tectonics are too complicated for that. Thus in the Piper Field in the Witch Ground Graben the lowest Upper Cretaceous is Coniacian which rests on Albian (Williams et al. 1975), yet there are places on the Fladen Ground Spur where the Upper Cretaceous is complete, albeit thin. When preserved, the Cenomanian is more argillaceous than the Lower Chalk in southeast England but there is no littoral or inner neritic facies; the benthic foraminifera indicate the outer rather than inner shelf zone. Burnhill & Ramsay's (1981) valuable analysis of the mid Cretaceous of this region indicates that the local absences of Cenomanian results from post-Cenomanian erosion rather than non-deposition. For the same reason the Cenomanian extended much further into the inner Moray Firth Basin than its present southwestern limit. Facies distribution in the remainder of the North Sea, and over onshore England, are based on sources quoted in Hancock (1986) and Rawson et al. (1978) respectively, plus released wells. Chalks are widely distributed in the Paris Basin (Juignet 1980; Robaszynski 1981). In the Haldon Hills and in south Devon the Cenomanian is arenaceous (Hamblin & Wood 1976; Hart et al. 1979); it extends no further with certainty than the underlying arenaceous Albian, and may never have done so. The general limits of land off the Cornish coast are based on Lott et al. (1980) and Evans et al. (1981), except south- and southwest of Plymouth which takes account of the records of Andreieff et al. (1975). The influence of the Cornubian Platform is known to extend far out into the Western Approaches, for in quadrants 72 and 73 the base of the Chalk is Santonian in several places, e.g. 72/10-1 (Bennet et al. 1985), and rests directly on Lower Cretaceous or older rocks. There may well be other areas over both the Cornubian Platform and Pembroke Arch with a similar break waiting to be discovered. The probability of more land than shown in the Western Approaches is supported by the distribution of facies. Although the Cenomanian sequence includes chalk or chalky limestone, it is also more glauconitic and richer in detritus than one finds in eastern England; occasionally the limestone is dolomitic. Even as far out as the Fastnet Basin part of the Cenomanian is arenaceous, deposited 'in a nearshore inner shelf environment' (Robinson et al. 1981). In the middle of the Western Approaches Basin between Cornwall and Brittany the lower half of the Cenomanian is a glauconie-facies, comparable to that in Antrim. Lott et al. (1980) record calcite cemented sandstones in the Lower Cenomanian on the margins of the Cornubian Landmass. To the northwest of the Cornubian Platform, the absence of Cenomanian sediment in some areas may be the result of erosion during a later tectonic uplift, enhanced by the late Turonian fall in sea level. Through parts of blocks 63/4 and 63/8 in the Fastnet Basin all the Chalk has been removed, even though the original Upper Cretaceous succession there was probably complete (Robinson et al. 1981). Similarly, the absence of all Upper Cretaceous in the East Bristol Channel Basin is no evidence for any original breaks in the succession there, keeping in mind the evidence for Tertiary inversion uplift, combined with a complete Upper Cretaceous succession only 100 km to the west in the Main Bristol Channel Basin between the Cornubian Platform and the Pembroke Arch (Kammerling 1979). In County Antrim the Lower Cenomanian is the glauconite-rich glauconie facies which was not formed near the shoreline; its western limit is faulted and higher Cretaceous sediments contain Cenomanian material derived from former more westerly extensions of the Cenomanian (Hancock 1961). Thus one cannot assume that the Cenomanian sea did not spread to places where one now finds higher stages of the Cretaceous resting directly on preCretaceous rocks. The neritic facies of the little scattered patches of Cenomanian amongst the Inner Hebrides all indicate that the coastline was not far to the east of their present occurrences. The heavy minerals also suggest derivation from Torridonian sediments to the east, and there is no unequivocal evidence for any material derived from the west or northwest; on the map here it is assumed that the Outer Hebrides were submerged. However, there is no surviving Cretaceous in the Minches between the Outer Hebrides and Skye where the solid geology is largely Jurassic (Chesher et al. 1981), nor even in the far northeast of Skye where there is still a protective basaltic cover (Anderson & Dunham 1966), and some authors, e.g. Ziegler (1982), consider this region to have been land throughout the late Cretaceous. To the west of the Shetland Platform, the Campanian rests directly on Upper Palaeozoic over the higher parts of the Rona Ridge (Ridd 1981; Mudge & Rashid 1987). Ziegler (1982) shows the Ridge submerged in both his Aptian-Albian and Cenomanian-Danian reconstructions. According to Ridd (1981) it was a positive area during the early Cretaceous and there was only 'a pulse of slight uplift on the Rona Ridge during the Late Cretaceous'. On balance it is more likely that there was an elongate island here during the Cenomanian (Hitchen & Ritchie 1987). When present on the Rona Terrace and the northeastern West Shetland Basin, the Cenomanian consists of limestones with thin glauconitic sandstones passing upwards into calcareous sandstones (Mudge & Rashid 1987). JMH K4b: L a t e C a m p a n i a n The late Campanian was the time of the highest sea levels during the
Cretaceous, with an exceptional monotony of facies: everywhere, except in the Viking Graben and Faeroe-West Shetlands Troughs, there was only chalk in the sea, and there are no known terrestrial sediments in the region. There are still considerable uncertainties on the extent of land. There was certainly much less than is shown on the Cenomanian-Danian palaeogeography of Ziegler (1982, enclosure 23), but the evidence varies in strength from area to area. The Scandinavian Landmass has been extended beyond the present Norwegian coast because there is no evidence at all that any Cretaceous sedimentation reached any of that part of present-day Norway shown on the map. The nearest Mesozoic onshore in Norway is at Andoy, latitude approximately 69~ and this does not include any Upper Cretaceous (Dalland 1975). There are Upper Campanian sediments in southern Sweden but they are at low elevations and include original shoreline facies (Surlyk & Christensen 1974). In Scotland there is evidence that the late Campanian sea encroached on to present-day land areas. In western Scotland there is chalk, in various conditions, on Skye, Mull and Arran, and on the mainland at Beinn Iadain 9 in Morvern; at this last locality there is a little evidence that the Chalk is Upper Campanian (Pringle & Manson 1934). To the east, the Buchan Ridge Gravels north of Aberdeen have been interpreted in various ways (McMillan & Merritt 1980: Kesel & Gemmell 1981) but it is difficult to explain their considerable content of flints mixed with local quartzites from Banff except as derivation from a former Cretaceous cover (Hall 1982). Nevertheless, the evidence is very thin and there is none to give them a Campanian date. The view that the whole of the Highlands were submerged by the late Cretaceous sea (Linton 1951) is rejected. It would simply imply a broad post-Cretaceous uplift of the Highlands by some 600 or more metres compared with the surrounding areas. The total absence of Cretaceous sediments over or near the Southern Uplands and Lake District provides very slight evidence that they were land. No pre-Oligocene erosion surfaces have been recognized in Bowland Forest (Moseley 1961). For Wales too there is a lack of positive evidence, but the probability that much of the area was still land is enhanced by the absence of all Cretaceous sediment in Cardigan Bay and most of the Irish Sea, as predicted by Blundell et al. (1971) and confirmed by B a r r e t al. (1981). Tucker & Arter (1987) believe the Cardigan Bay Basin was covered by Upper Cretaceous sediment which has since been eroded. Further south, analysis of the fossil assemblages in the gravels on the Haldon Hills and at Orleigh Court in north Devon show that normal chalk up to the Upper Campanian was deposited in Devon. When allowance is made for probable original thicknesses and assuming that the Chalk sea was 200-300m deep, the whole of Cornubia would have been submerged (Hancock 1969). The same is probably true for Armorica, where the highest land today is only 340 m. There are marine Oligoeene sediments in the Rennes Basin and it should have been even easier for the Upper Cretaceous to have spread over Brittany. One has to admit that there are no residual sediments over the core of the massif. In the Ardennes there are flint residuals at 545-560m on Cambrian southeast of Liege. They are dated as Maastrichtian, probably mid Maastrichtian. Although there has been a considerable amount of Cenozoic faulting and tilting (Robaszynski 1981) there seems no reason to believe that there was any land left at the height of the late Campanian transgressions. Most of Ireland was probably submerged. There is a considerable spread of Upper Campanian Chalk in Northern Ireland (Hancock 1961; Reid 1971). On the coasts of north and east Antrim it comes down to the present sea level but inland to the southwest it rises topographically until it reaches over 300 m in Co. Derry east of Dungiven, and in Co. Tyrone on Slieve Gallion west of Magherafelt. Not too much importance should be attached to exact heights because of the amount of Cenozoic faulting, but there is a general rise westwards. Nor is there any sign that the Chalk could not have spread further. The discovery by Walsh (1966) of outliers of Upper Campanian Chalk in Co. Kerry confirms the widespread extent. There may have been islands amongst the high ground of Kerry and Cork but the presentday heights are comparable to those of Donegal which was almost certainly submerged (Dury 1959). On the assumption of a Cenozoic tilt of Ireland downwards to the east, the Wicklow Mountains would have been land during the late Campanian. In the northwest part of the map area there is evidence of minor late Cretaceous igneous activity (summarized by Hitchen & Ritchie 1987). It includes possibly Campanian vulcanicity on the Anton Dorhn Seamount and radiometrically dated igneous intrusions in two wells along the eastern side of the Faeroe Rift (see map). JMH
References References not listed here are cited in Rawson et al. (1978). AINSWOR/H, N. R., O'NEtLL, M., RUTHERFORD,M. M., CLAVVON,G., HORTON, N. F. & I~NNV, R. A. 1987. Biostratigraphy of the Lower Cretaceous, Jurassic and Late Triassic of the North Celtic Sea and Fastnet Basins. In: Bgoogs, J. & GLENNIE, K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London. 611-622. ALLEN, P. 1981. Pursuit of Wealden models. Journal of the Geological Society, London, 138. 375~405. AMIm-GARROUSSl,K. 1988. Sedimentology and reservoir potential of a "marginal facies" limestone from the Cretaceous of the North Sea. Marine and Petroleum Geology, 5, 182-192.
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ANDERSON, F. W. & DUNHAM,K. C. 1966. The Geology of Northern Skye. Memoirs of the Geological Survey, United Kingdom. BARR, K. W., COLTER,V. S. & YOUNGR. 1981. The geology of the Cardigan BaySt George's Channel Basin. In: ILLING,L. V. & HoasON, G. D. (eds) Petroleum Geology of the Continental Shelf of North- West Europe. Heyden, London, 432443. BENNET, B., COPESTAKE,P. & HOOKER, N. P. 1985. Stratigraphy of the Britoil 72/ 10-1A well, Western Approaches. Proceedings of the Geologists'Association, 96, 255-261. BLUNDELL, D. J., DAVEY,F. J. ,ge GRAVES,t. J. 1971. Geophysical surveys over the south Irish Sea and Nymphe Bank. Journal of the Geological Society, London, 127, 339 375. BOOTE, D. R. D. & GUSTAV,S. H. 1987. Evolving depositional systems within an active rift, Witch Ground Graben, North Sea. In: BROOKS,J. & GLENNIE,K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 819-833. BURNHILL, T. J. & RAMSAY, W. V. 1981. Mid-Cretaceous palaeontology and stratigraphy, central North Sea. In: ILLING,L. V. & HOBSON,G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 245-254. CHESHER,J. A., SMYTHE,n. K. &BISHOP, P. 1981. The Geology of the Minches, Inner Sound and Sound of Raasay. Institute of Geologial Sciences, Report g3/6. COPE, J. C. W. 1984. The Mesozoic history of Wales. Proceedings of the Geologists' Association, 95, 373-385. CRITTENDEN,S. 1982. Lower Cretaceous lithostratigraphy NE of the Sole Pit area in the UK southern North Sea. Journal of Petroleum Geology, 5, 191-202. DALLAND, A. 1975. The Mesozoic rocks of Andoy. northern Norway. Norges Geologiske Undersokelse 316, 271-287. DAY, R. A., COOPER, B. A., ANDERSEN,C., BURGERS,W. F. J., RONNEVIK,H. C. & SCHONEICH,H. 1981. Regional seismic structure maps of the North Sea. In: ILLING, L. V. & HOBSON,G. D. (eds) Petroleum Geology of the Continental Shelf o/North-West Europe. Heyden, London, 76-84. DEEGAN,C. E. & SCULL, B. J. 1977. A Standard Lithostratigraphic Nomenclature for the Central and Northern North Sea. Institute of Geological Sciences, Report 77/25. DIXON, J. E., FITTON,J. G. & FROST,R. T. C. 1981. The tectonic significance of postCarboniferous igneous activity in the North Sea Basin. In: ILLING, L. V. & HOBSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North- West Europe. Hey~en, London, 121-137. DURY, G. H. 1959. A contribution to the geomorphology of central Donegal. Proceedings of the Geologists" Association, 70, 1-27. EVANS, C. O. R., LOTT, G. K. & WARRINGTON,G. 1981. The Zephyr (1977) Wells, South-Western Approaches and Western English Channel. Institute of Geological Sciences, Report 81/8. EVANS, D., CHESHER,J. A., DEEGAN,C. E. & FANNIN,N. G. T. 1982. The Offshore Geology of Scotland in Relation to the IGS Shallow Drilling Programme, 19701978. Institute of Geological Sciences, Report 81/12. FORD, T. D. & KING, R. J. 1968. Outliers of possible Tertiary age. In: SYLVESTERBRADLEY,P. C. & FORD,T. D. (eds) The Geology of the East Midlands. Leicester University Press, Leicester, 324-331. FYFE, J. A., AnSOTTS, I. & CROSBY, A. 1981. The subcrop of the mid-Mesozoic unconformity in the UK area. In: ILLING, L. V. & HOBSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 236--244. GAIDA,K-H., KEMPERE. & ZIMMERLE,W. 1978. Das Oberapt yon Sarstedt und Seine Tuffe. Geologisches Jahrbuch, A45, 43-123. GLENNIE, K. W. & BOEGNER,P. L. E. 1981. Sole Pit inversion tectonics. In: ILLING, U V. & HOBSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North- West Europe. Heyden, London, 110-120. HALL, A. M. 1982. The "Pliocene" gravels of Buchan: a reappraisal: discussion. Scottish Journal of Geology, 18, 336-338. HANCOCK,J. M. 1975. The sequence of facies in the Upper Cretaceous of northern Europe compared with that in the western interior. In: CALDWELL,W. G. E. (ed.) The Cretaceous System in the Western Interior of North America. The Geological Association of Canada, Special Paper, 13, 83-118. -1986. Cretaceous. In: GLENNIE, K. W. (ed.) Introduction to the Petroleum Geology of the North Sea. Blackwell, Oxford, 161-178. HARKER, S. D., GUSTAV, S. H. & RILEY, L. A. 1987. Triassic to Cenomanian stratigraphy of the Witch Ground Graben. In: BROOKS,J. & GLENNIE, K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 809-818 HART, M. B., MANLEY,E. C. & WEAVER,P. P. E. 1979. A biometric analysis of an Orbitolina fauna from the Cretaceous succession at Wolborough, S. Devon. Proceedings of the Ussher Society, 4, 317-326. HERNGREEN, G. F. W., SMIT,R. & WONG,Th. E. 1989. Upper Jurassic-Cretaceous stratigraphy of the Vlieland Basin, and adjacent areas, the Netherlands. Proceedings of a Symposium on the Jurassic in the southern Central Graben, Horsholm (Denmark), June 15-16 1989, abstract. - & WONG, Th. E. 1989. Revision of 'Late Jurassic' stratigraphy of the Dutch Central North Sea Graben. Geologie en Mijnbouw, 68, 73-105. HESJEDAL,A . 8/. HAMAR,G. P. 1983. Lower Cretaceous stratigraphy and tectonics of the south-southeastern Norwegian offsbore. Geologie en Mijnbouw, 62, 135-144. HEYBROEK, P. 1975. On the Structure of the Dutch Part of the Central North Sea Graben. In: WOODLAND,A. W. (ed.) Petroleum and the Continental Shelf of North-West Europe. Volume 1: Geology. Applied Science, Barking, 339-349. HITCHIN, K. & RITCHIE,J. D. 1987. Geological review of the West Shetland area. In: BROOKS,J. & GLENNIE,K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 737-749.
JEANS,C. V., MERRIMAN,R. J., MITCHELL,J. G. & BLAND,D. J. 1982. Volcanic clays in the Cretaceous of southern England and Northern Ireland. Clay Minerals, 17, 105-156. JENSEN, T. F., HOLM, L., FRANDSEN,N. & MICHEt.SEN, O. 1986. Jurassic-Lower Cretaceous lithostratigraphic nomenclature for the Danish Central Trough. Danmarks Geologiske Undersogelse, Serie A, 12. JUIGNET. P. 1980. Transgressions-rEgressions, variations eustatiques et influences tectoniques de l'Aptien au Maastrichtien dans le Bassin de Paris occidental et sur la bordure du Massif Armoricain. Cretaceous Research, 1, 341-357. KAMMERLING,P. 1979. The geology and hydrocarbon habitat o f the Bristol Channel Basin. Journal of Petroleum Geology, 2, 75-93. KARLSSON,W. 1986. The Snorre, Statfjord and Gullfaks oilfields and the habitat of hydrocarbons on the TampeD Spur, offshore Norway. In: SPENCER,A. M. et al. (eds) Habitat of Hydrocarbons on the Norwegian Continental Shelf. Graham & Trotman, London 181-197. KAYE, P. 1966. Lower Cretaceous Palaeogeography of North-West Europe. Geological Magazine, 103, 257-262. KESEL, R. H. & GEMMELL,M. D. 1981. The "Pliocene" gravels of Buchan: a reappraisal. Scottish Journal of Geology, 17, 185-203. LINTON, D. L. 1951. Problems of Scottish scenery. Scottish Geographical Magazine, 67, 65-85. LOTT,G. K., BALL,K. C. • WILKINSON,1. P. 1985. Mid Cretaceous stratigraphy o f a cored borehole in the western part of the Central North Sea Basin. Proceedings of the Yorkshire Geological Society, 45, 235-248. - - , FLETCHER, B. N. & WILKINSON, 1. P. 1986. The stratigraphy of the Lower Cretaceous Speeton Clay Formation in a cored borehole off the coast of northeast England. Proceedings of the Yorkshire Geological Society, 46, 39-56. , KNOX, R. W. O'B., BIGG,P. J., DAVEY,R. J. & MORTON,A. C. 1980. AptianCenomanian Stratigraphy in Boreholes from Offshore South-west England. Institute of Geological Sciences, Report 80[8. MCMILLAN, A. A. & MERRI'Vr,J. W. 1980. A Reappraisal of the "Tertiary" Deposits of Buchan, Grampian Region. Institute of Geological Sciences, Report 80/1, 1825. MEADOWS, N. S., MACCHI, L., CUBITT,J. M., & JOHNSON, B. 1987. Sedimentology and reservoir potential in the west of Shetland, UK exploration area. In: BROOKS, J. & GLENNIE,K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 723-736. MEGNIEN, C. & MfGNIEN, F. (eds) 1980. Synth~se grologique du bassin de Paris. Mbmoires du Bureau de Recherches Gdologiques et Minidres, 1 0 1 - 1 0 3 . MIDDLEMISS, F. A. 1961. Brachiopods and shorelines in the Lower Cretaceous. Annals and Magazine of Natural History, ser. 13 (4), 613-626. MOSELEY, F. 1961. Erosion surfaces in the Forest of Bowland. Proceedings of the Yorkshire Geological Society, 33, 173-196. MUDGE,D. C. & RASHID,B. 1987. The geology of the Faeroe Basin area. In: BROOKS, J. d~.GLENNIE,K. W. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 751-763. NAM 1980. Stratigraphic nomenclature of the Netherlands. Nederlandse Aarodolie Maatschappij B. V. and Rijks Geologische Dienst. PEACH, B. N., GUNN, W. & NEWTON, E. T. 1901. Remarkable volcanic vent of Tertiary age in the Island of Arran, enclosing Mesozoic fossiliferous rocks. Quarterly Journal of the Geological Society, London, 57, 226-243. RAWSON, P. F. 1991. The Cretaceous. In: SMITH, A. J. & McDo~, L. (eds) Geology of England and Wales. Geological Society, L o n d o n . 9 CURRY, D., DILLEY, F. C., HANCOCK,J. M., KENNEDY,W. J., NEALE,J. W., WOOD, C. J. & WORSSAM,B. C. 1978. A Correlation of Cretaceous Rocks in the British Isles. Geological Society, London, Special Report 9. - & RILEY, L. A. 1982. Latest Jurassic-Early Cretaceous events and the "Late Cimmerian Unconformity" in North Sea area. Bulletin of the American Association of Petroleum Geologists, 66, 2628-2648. RIDD, M. F. 1981. Petroleum geology west of the Shetlands. In: ILLING, L. V. & HOBSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North- West Europe. Heyden, London, 414-425. ROBASZYNSKI,F. 1981. Moderation of Cretaceous transgressions by block tectonics: an example from the north and north-west of the Paris Basin. Cretaceous Research, 2, 197-213. ROBINSON, K. W., SHANNON,P. M. & YOUNG,D. G. G. 1981. The Fastnet Basin: an integrated analysis. In: ILLING,L. V. & HOBSON,G. D. (eds) Petroleum Geology of the Continental Shelf of North- West Europe. Heyden, London, 444--454. SKJERVEN, J., RIJS, F. & KALHEIM,J. E. 1983. Late Palaeozoic to Early Cenozoic structural development of the south-southeastern Norwegian North Sea. Geologie en Mijnbouw, 62, 35--45. SURLYK, F. & CHRISTENSEN,W. K. 1974. Epifaunal z o n a t i o n o n a n Upper Cretaceous rocky coast. Geology, 2, 529-534. TUCKER, R. M. & ARIrR, G. 1987. The tectonic evolution o f the North Celtic Sea and Cardigan Bay basins with special reference to basin inversion. Tectonophysics, 137, 291-307. VAN WIJKE, D. H. 1987. Structural evolution of inverted basins in the Dutch offshore. Tectonophysies, 137, 171-219. WALSH, P. T. 1966. Cretaceous outliers in south-west Ireland and their implications for Cretaceous palaeogeography. Quarterly Journal of the Geological Society, London, 122, 63-84. WILLIAMS,J. J., CONNER, D. C. & PETERSON,K. E. 1975. The Piper oil-field, U.K. North Sea: a fault-block structure with Upper Jurassic beach-bar reservoir sands. In: WOODLAND,A. W. (ed.) Petroleum and the Continental Shelf of Northwest Europe. Volume 1: Geology. Applied Science, Barking, 363-377. ZIEGLER, P. A. 1982. Geological Atlas of Western and Central Europe. Shell International Petroleum Maatschappij B. V. -1988. Evolution of the Arctic-North Atlantic and the Western Tethys. American Association of Petroleum Geologists, Memoir, 43.
139
Palaeogene and Neogene J. W. MURRAY
Most of the British Isles has been a land area throughout the Tertiary and the basic shape of the coastline had been determined by the late Palaeocene. Only in southeast England are there marine Palaeogene and Neogene successions and these were deposited in embayments marginal to the North Sea and English Channel. Much of the key information for the interpretation of the Tertiary history of the British Isles lies offshore. Thousands of kilometres of seismic profiles have been run and hundreds of boreholes drilled, mainly in the search for oil. The only part of the Tertiary which has been of economic interest is the Palaeocene of the northern North Sea. Summaries of the offshore data are given in Woodland (1975) and Illing & Hobson (1981). Thick Tertiary successions are known from the central Graben of the North Sea, the Rockall Trough and the Faeroe Basin (Map Pg 1). The North Sea was the site of great subsidence caused by thermal relaxation following stretching of the continental crust (Sclater & Christie 1980). None of the Tertiary sediments was deposited in very deep water. By contrast, the Rockall Trough and the Faeroe Basin are underlain by oceanic crust and deep water has existed there throughout the Tertiary. The successions are known only from seismic studies (Roberts 1975) but are thought to be mainly fine-grained and with some contourites. On a regional scale, the Tertiary history of northwest Europe must be considered in relation to the plate tectonic events proceeding in the adjacent ocean. The Rockall Trough had opened through sea floor spreading by the end of the Cretaceous (Roberts 1975). The northern North Atlantic Ocean commenced opening in the Early Eocene (Anomaly 24B, Roberts et al. 1984) and so too did the Norwegian Sea (Talwani & Udintsev 1976). The crossocean barrier formed by the Greenland-Iceland-Faeroes-Scotland Ridge may have been an obstacle to deep water circulation in the Early Palaeogene but there can be no doubt that shallow water connections existed between the Rockall Trough and the Faeroe Basin throughout the Tertiary. Also, a connection existed between the Arctic Ocean and the North Sea via the Norwegian-Greenland Sea. Global events, including eustatic changes of sea level and climatic variation, also influenced the Tertiary development of the British Isles. The deep sea isotopic record shows a progressive cooling of surface waters during the Cenozoic (Berger et al. 1981). Higher latitudes were affected more than the tropics. Transgressions were linked with ocean warming, an extension of the tropical zone, and higher rainfall. Regressions correlate with ocean cooling and regional drought. Superimposed on the overall temperature drop there were 'steps', the most noteworthy of which were the introduction of cold water to the abyssal ocean at the Eocene-Oligocene boundary, the Middle Miocene build-up of ice on East Antarctica, and the Late Pliocene initiation of northern hemisphere glaciation. PALAEOGENE
Pgl: Palaeocene The Palaeocene-Early Eocene igneous rocks of the British Isles form part o f the much larger Thulean igneous province which extends along the eastern seaboard of Greenland, the western margin of Rockall Plateau, through the Faeroes and Iceland, the Rockall Trough and the Faeroe Basin. In western Scotland, the land surface prior to the eruption of the lavas was subaerial with lowland relief and localized alluvial deposits (George 1966). Conditions were probably similar in Northern Ireland as the thin Cretaceous successions were preserved beneath the lavas thus showing little or no prevolcanism erosion (George 1967). The thickness of volcanic deposits reached a maximum of 3000 m and at their greatest development the volcanoes and lava fields must have formed regions of substantial relief. The 2.5 km vertical difference in elevation of the base of the lava piles is thought to be due to post-eruption warping and faulting (Watson 1985). Away from the main area of volcanic activity, the Lundy granite was emplaced adjacent to the Sticklepath fault and intrusive bodies were emplaced in the Fastnet area of the Celtic Sea (Caston et al. 1981). In southwest England uranium mineralization took place (Edmunds et al. 1969). 9 1992The GeologicalSociety
The Laramide orogeny, from Late Cretaceous to Early Palaeocene, caused widespread vertical movements throughout northwest Europe (Ziegler 1987). This led to the start of the inversion of the Mesozoic basin of the Weald to give rise to the Weald-Artois high, and also to the Celtic Sea and Bristol Channel basins. Seismic records from the Celtic Sea show Palaeogene deposits apparently conformable on chalk and, if this is the case, the early Palaeocene must be represented (Naylor & Mounteney 1975). There is no other evidence of early Palaeocene deposits along the western shelf of the British Isles. In the Rockall Trough they form part of the pre-R4 series (Roberts 1975). On the eastern margin of the Faeroes Basin there are dark greenish-grey or brown mudstones with lignite bands. These beds, containing reworked Jurassic spores thought to have been derived from the uplifted Shetland Platform (Ridd 1981), are underlain by a prominent seismic reflector thought to represent the top of a lava sequence which correlates with the early development of the Faeroe Islands basaltic fissure eruptions. The original extent of early Palaeocene deposits in the North Sea was much greater than that now seen as erosion has subsequently removed them from everywhere except the graben areas (Pegrum et al. 1975; Knox el al. 1981). Following the deposition of the early Palaeocene (Danian) chalks and bioclastic limestones there was a major regression which led to extensive erosion of these deposits. Further erosion took place during the transgressive advance of the Thanet Formation sea into the London Basin. Map Pg 1 portrays the geography during the subsequent Reading Formation times. The whole of the British Isles, with the exception of southeast England, was emergent and there is strong evidence that uplift was at a maximum in the northwest. The regional tilt was to the east or southeast. From heavy mineral studies it can be seen that there were important sediment source areas in the Shetland Platform and Scottish Highland areas (Knox e t al. 1981) and these supplied not only the graben regions of the North Sea, but also the Shetland Shelf prograding into the Faeroe Basin (Ridd 1981). Much of England may have been of fairly low relief, perhaps with drainage to the southeast. Longshore drift transported detritus from the Scottish Highlands to the London and Hampshire basins and there was also an input from the Armorican and possibly Ardennes areas (Morton 1982a,b). There is no evidence of a supply from southwest England at this time. Isopachs on both the Thanet and Reading formations suggest that the We,alden area was a positive feature, even if of low relief, in part dividing the depositional area into two basins. In the northern North Sea, the succession consists of the Montrose and Moray groups, the latter including tuff horizons which form excellent markers on seismic profiles. Details of the Montrose Group have been plotted on~the map (based on Rochow 1981). The commencement of clastic sedimentation in the Middle Palaeocene was the result of substantial uplift of the Shetland Platform and Scottish Highlands, the development of drainage from these areas towards the east, and contemporary subsidence with associated downfaulting in the North Sea, especially in the Viking Graben, Moray Firth Basin and Central Graben. Marine mudstones and sandstones accumulated on a shelf platform southeast of the Orkneys, but much of the sediment bypassed this shallow area to be deposited as submarine fans in the graben basins. Most of this succession consists of sands deposited from turbidity currents and mass flow. Growth faulting and compaction kept pace with deposition during Montrose Group accumulation (although this was not the case in the succeeding Moray Group). The basinal areas are thought to have been of modest depth, a few hundred metres (Knox et al. 1981) or 600-900m (Parker 1981). Sediment direction arrows show that deposition was outward from west to east into the basins but in the axial regions sediment was transported to the south and southeast. Thus, the sand tongue in the Central Graben was supplied by axial transport from the Moray Firth Basin. Outside these basinal areas, there was little subsidence and thin successions of mainly mudstone facies were deposited. Towards the end of deposition of the Montrose Group palaeoslopes became gentle as sedimentation exceeded subsidence. During the deposition of the Moray Group growth faulting ceased. The basins filled along their western margins and shelf and deltaic deposits prograded to the east. The sediments are silty shales and become lignitic in the Moray Firth area. This indicates silting up of the basin to form a plain of low relief. Heavy minerals 141
142
PALAEOGENE AND NEOGENE
have been studied to determine provenance (Knox et aL 1981; Morton 1982c), with identification of three souce areas, the metamorphic basement of the northern Shetland Platform, the sediments of the southern Shetland Platform, and the metamorphic rocks of the Scottish Highlands. These source areas are thought to have alternated in importance throughout the deposition of the Montrose and Moray Groups (Morton 1982c). The final (transgressive) event of the Palaeocene was the deposition of laminated clays and tufts. The latter are thought to have been derived from the west of Scotland by northwesterly winds (Jacqu6 & Thouvenin 1975). In the southern North Sea claystones accumulated in relatively shallow water (Pegrum et al. 1975). In the London Basin the Thanet Formation, consisting of fine sands, siltstones and claystones, reaches thicknesses of up to 30 m (Hester 1965). The presence of heavy mineral associations similar to those of the Moray Firth area is thought to indicate a sediment source, in part, from the Scottish Highlands, with transport in the North Sea by longshore drift (Knox et al. 1981; Morton 1982a). The Reading and Woolwich formations overstep the Thanet Formation in the London Basin. In the Hampshire Basin the Thanet Formation is absent. The Reading Formation is well developed, but only around Newhaven and the Sussex coast do deposits of Woolwich type occur. The typical Reading Formation is mottled red and green clay but locally there are sand and pebble beds. The Bottom Bed contains a heavy mineral assemblage thought to be derived from the Armorican or Ardennes-Rhenish massifs to the south (Morton 1982a,b). Palaeosols within the Reading Formation are hydromorphic, representing deposition in a warm climate with a marked dry season, possibly under fluviomarine conditions (Buurman 1980). The whole of southeast England must have been an area of extensive marshes with watercourses, an oscillating coastline with a sand barrier and a tidal inlet complex (Ellison 1983). The Woolwich Formation is sandy in the lower part and a dark fossiliferous clay in the upper part; the fossils indicate a brackish environment. In the Channel, offshore from Dieppe to Fecamp and on land west of Dieppe, there is a white microcrystalline or argillaceous limestone (of presumed Thanetian age) resting unconformably on chalk (Auffret et al. 1975). Above this are 'Sparnacian' muddy sands and lignites. Reading Formation mottled clays and sands occur in the Central Channel. In the Western Channel Boreholes Zephyr 87/16 and 87/14-1 have probable Late Palaeocene limestones (Evans et al. 1981). In the Celtic Sea there are volcanic rocks possibly of Palaeocene age (Caston et al. 1981). Late Palaeocene to Early Eocene uplift south of Ireland reached 1000m along inversion axes (Tucker & Arter 1987). The Faeroe Basin is largely unexplored. Seismic results show a wedge of sediments along the margin of the Shetland Platform, prograding to the northwest. Limited borehole information from west of Shetland shows the lithologies to be clastic (mudstone, siltstone, sandstone) and the top Palaeocene tufts form a distinctive marker horizon (Ridd 1981). The source of the tuff could be the Faeroes area if the prevailing winds were from the northwest as suggested by Jacqu6 & Thouvenin (1975). In the deeper parts of the basin there is a prominent seismic reflector which is thought to represent the top of a Palaeocene lava sequence. In the axis of the basin there is a possible chain of igneous centres (distribution of igneous rocks plotted from Ziegler 1981). The southwest termination of the Faeroe Basin is the Wyville Thomson Ridge, an anticlinal structure of Palaeogene volcanic rocks. Widespread subaerial extrusion of basalts in this area may have formed a land bridge to Greenland but, in all probability, there was a shallow open seaway between the Rockall Trough and the Faeroe Basin which allowed an interchange of marine faunas between the North Atlantic and the North Sea. The Rockall Trough has a thick Cenozoic sediment fill. Palaeocene sediments are not separately differentiated but are a part of the pre-R4 units (Roberts 1975). Reflector X within this sequence may be of Palaeocene age. The distinctive features about the sedimentary record along the western margin of Scotland and Ireland are that only a thin succession is present on the shelf margin, the shelf edge is a fault line, and the thicker successions occur within the Trough.
PALAEOCENE-EOCENE
OF SOUTHERN
ENGLAND
Pg2a: Early Palaeocene and the sub-Palaeocene floor The inversion of the Weald led to uplift of 300-450 m and to the loss of ,,~200 m of chalk through erosion by the Early Palaeocene (Jones 1980). The distribution of Cretaceous rocks beneath the Palaeogene unconformity shows that the axis of uplift does not coincide with the region of greatest erosion of chalk (Curry, 1965, and pets. comm.). The early Palaeocene (Danian) deposits in the English Channel are bioclastic limestones resting conformably on Maastrichtian chalk (Evans et al. 1981). Thus carbonate sedimentation continued across the CretaceousTertiary boundary. However, it was limited to the basinal area of the western Channel. The presence of planktonic foraminifera is evidence of communication with the open ocean to the west. In the Paris Basin, bioclastic limestones containing calcareous algae, molluscs and corals were deposited in shallow (< 15 m), clear, warm-temperate waters. In east Devon there are in situ residual deposits resulting from the dissolution of chalk,
for example the Combpyne Soil (Isaac 1981) and Tower Wood Gravels (Hamblin 1973) which may be partly of Palaeocene age.
Pg2b: Early Eocene The map shows the interpreted palaeogeography during London Clay Formation times with the shoreline plotted for the lower part of division Bl (using the nomenclature of King 1981). Knox & Harland (1979) interpret the London Clay Formation transgression as coming from the northern part of the North Sea and advancing diachronously into southeastern England and then westwards into the Hampshire Basin (Knox et al. 1981; pers. comm.). The present occurrence of the Formation is in two separate areas; the London Basin and the Hampshire-Dieppe Basin which are remnants of a once continuous basin of deposition. The sequences show five transgressive-regressive sedimentary cycles. The environments of deposition and the position of the shorelines migrated backwards and forwards during these cycles (see Murray & Wright 1974, for details). Studies of heavy minerals show that a Scottish Highlands source supplied material throughout the Early Eocene. Transport was probably by longshore drift southwards. In the upper part of the London Clay Formation in the Hampshire Basin, minerals having an Armorican source are present (Morton 1982a,b). In the overlying Wittering Formation there are minerals derived from Cornubia and this led Morton (1982b) to suggest uplift of this area. In southwest England, flint gravels derived from the chalk are believed to be of early Eocene age (Isaac 1983). Fine-grained bioclastic limestones are known from the Western Channel while in the Western Approaches there are both limestones and sandstones (Evans et al. 1981). The gulf of the sea occupying the Western Channel was an area of open marine water (Bennet et al. 1985) whereas the seas in the Hampshire Basin were shallow, turbid, and somewhat brackish from time to time. In the London Basin similar marginal marine conditions were developed especially in the west. The existence of river input is shown by the presence of seeds and fragments of land plants. To the east in Essex, the water was probably deeper, perhaps > 200 m, as shown by the abundance and size of the planktonic foraminifera.
Pg2c: Mid Eocene During the mid Eocene, the North Sea may have been intermittently isolated from the Hampshire-Paris-Channel areas by the elevated WealdArtois shoal or low-lying land mass. Sand deposition took place in the Belgian Basin (Brussels, Lede and Wemmel, in part), and Jn the London Basin (Bagshot 'Sands', in part). Heavy minerals from the latter indicate a Scottish Highlands provenance (Morton 1982a). In the Hampshire Basin this period is represented by part of the Bracklesham Group (top of the Earnley Formation, Marsh Farm, Sr Sand, and lowermost Barton Clay Formations, and their lateral equivalents). Much of the area was the site of deposition of non-marine sediments but in the latter part of the mid Eocene a shallow marine embayment developed (Murray & Wright 1974). There is evidence of contemporaneous uplift of the precursor of the Isle of Wight monocline with flint pebbles being transported into the basin to the north, and the Portsdown anticline may have been initiated at this time (Plint 1982, 1983). The glauconitic silty and sandy clays of the Eastern Channel are similar to the marine deposits of the Hampshire Basin (Curry & Smith 1975). In the Paris Basin and Western Channel, limestone accumulated (Evans et al. 1981). Small relicts of similar deposits on the Cotentin show that an arm of the sea extended across it. The rich microfaunas of the carbonate sediments show them to have been deposited in very shallow water, with submarine vegetation (?seagrass) in places, a temperature of > 22~ in the summer and salinity of 36-40%o (Murray & Wright 1974). If the Weald-Artois region formed an emergent barrier at this time, the sea would have been a shallow, almost land-locked gulf open to the Atlantic Ocean only through the Western Approaches. It seems likely that there was a dry climate to the south, with very little run-off and therefore little sediment supply to the Western Channel and the Paris Basin. However, one or more rivers brought clastic detritus into the Hampshire and Eastern channel Basin and into the much larger North Sea Basin. The Sticklepath fault may have been active at this time, with sinistral movement leading to the initiation of the pull-apart basins of Bovey and Petrockstow in which fluvio-lacustrine deposits accumulated (Holloway & Chadwick 1986).
Pg2d: Late Eocene In the Hampshire Basin, the late Eocene includes the Barton and Solent Groups. These represent varied marginal marine and continental environments. The shoreline shown is that of the lagoon developed in the Headon Hill Formation (i.e. Middle Headon Beds). This varied in salinity from normal to brackish (Murray & Wright 1974). Reworked Headon Hill Formation (Lower Headon Beds) limestone suggests contemporary tectonic uplift (Murray & Wright 1974; Piint 1982, 1984). In the Eastern Channel there are deposits which are possibly equivalent to the Barton Group (Curry & Smith 1975). In the Western Channel there are
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PALAEOGENE AND NEOGENE
traces of bioclastic limestones (Curry et al. 1970) which were deposited in a gulf connected to the open Atlantic to the west. Between the Start Peninsula and the Cotentin there was a shoal or possibly even an intermittent land barrier. The isolation from the Belgian Basin of the Paris Basin was brought about by further uplift of the Weald-Artois barrier throughout the late Eocene (Pomerol 1982). The sediments are varied in kind and in environment, ranging from marine sands through brackish to hypersaline clays, freshwater limestones, soils, to evaporites (gypsum). In southwest England, the development of the Bovey and Petrockstow Basins continued. Drainage was southeast into the former and northwest into the latter (Edwards & Freshney 1982). The ball clays were derived from the weathering of Carboniferous shales rather than from the Dartmoor granite. The same is true of the recently discovered Dutson Basin (Freshney et aL 1982).
Pg3: Mid-Late Oligocene The latest Eocene-earliest Oligocene was the time of first appearance of cold deep water in the oceans and this was linked with a fall in sea level (Berger et al. 1981). The North Sea Basin was smaller in extent than at any other time and inversion took place in the Sole Pit area (Van Hoorn 1987) and the Netherlands (Van Wijhe 1987). On the limited data available most of the deposits appear to have been clays. There is no direct evidence of a sea connection between the North Sea and the Channel, although the molluscan faunas of the late Eocene Headon Hill formation (Brockenhurst Bed), Grimmertingen Sands (Belgium) and Latdorf (northwest Germany) have close affinities (Pomerol 1982). The succession of the Hampshire Basin comprises mainly non-marine fluvio-lacustrine deposits with only brief marine or near marine incursions (e.g. the Bouldnor Formation, Upper Hampstead Beds). The basin had almost completely silted up and there seems little doubt that it was becoming progressively isolated through tectonic activity. Inversion affected the Channel and southern part of the Isle of Wight (Ziegler }987). In the Paris Basin, much of the succession is lacustrine but there are also younger marine sands and brackish water clays so a connection with the open sea must have existed, if only during early Oiigocene times, via the Loire Valley (Pomerol 1982) or with the English Channel. To the west of the Channel Islands there are isolated outcrops that may be of Oligocene age. These include brackish clays, marine sandy limestones and freshwater limestones (Curry et al. 1965, 1970; Andreieff & Lefort 1972; Andreieff et al. 1975). This suggests the existence of a gulf in which conditions varied from shallow marine to lacustrine. To the west, borehole 72/10-1A penetrated marine calcareous muds with planktonic foraminifera of probable late Oligocene age (Bennet et al. 1985). Another shallow gulf occupied the Celtic Sea to the south of Ireland and here some 150 m of silty limestones were deposited (Colin et al. 1981). The main feature of mid to late Oligocene deposition onshore was the development of a number of lacustrine and fluvio-lacustrine basins. Some, such as the Bovey, Petrockstow and Dutson basins, had been initiated in the Eocene. Others (Stanley Bank, Mochras, Lough Neagh, Sea of the Hebrides and Canna basins) seem to be only of mid to late Oligocene age. In addition, small deposits such as those of St Agnes, Flimston and Ballymacadam, are believed to be of this age. Most of these areas are fault-controlled basins of deposition, and although their deposits are similar, they are to be regarded as localized depositional areas formed ,mder the same climatic conditions. Wilkinson et al. (1980) believed that the basins may have been linked by a river system shown diagrammatically in Map Pg3. Although the thickness of deposits was often considerable, deposition was under shallow water conditions (Edwards 1976) Thus, there was contemporaneous movement along fault lines with sedimentation equalling subsidence. Holloway & Chadwick (1986) believe the Petrockstow and Bovey basins to be pull-apart basins formed during sinistral movement of side-stepping transcurrent faults. In the Rockall Trough, reflector R4 is dated as late Eocene-late Oligocene by Roberts (1975) and is equated with a chert horizon especially in the deeper parts of the Trough. However, it should be noted that this has not yet been confirmed by drilling. The cherts may be indicative of upwelling or an elevated calcium carbonate compensation depth. Little is known of the Faeroe Basin deposits except that they prograde from the Shetland Platform into the basin (Ridd 1981). Itis likely that a sea connection existed with the Rockall Trough.
NEOGENE
Ngl: Miocene-Pliocene The UK outcrop limit of Neogene deposits is based on Woodland (1979). There are no Miocene deposits on land. Marine Pliocene deposits are known from St Erth (Cornwall), Lenham and adjacent areas (southeast England) and Suffolk (Coralline Crag). Terrestrial Pliocene deposits have been described from Brassington (Pennines). Erosion surfaces of supposed Miocene
and Pliocene ages have been described by many authors, but, with the exception of Brassington, none has been dated by reliable means. The Brassington deposits are preserved in karstic caverns. They are of early Pliocene age (based on palynology), and are thought to have formed on a surface which would now be at +450 m (Walsh et al. 1972). These authors also suggest that only a few tens of metres of material has been eroded from the Hercynian massifs since the Pliocene although erosion of softer Permian, Mesozoic and Tertiary deposits elsewhere may have been much greater. In France, the Sologne and Lozrre sands of the Seine Valley are considered to be continental in origin and of Miocene age (Pomerol 1982). Ziegler (1982) drew the Neogene shoreline along the present outcrop limit. This is not followed here because the majority of recorded deposits are not of shoreline type. The North Sea Basin continued to subside throughout the Neogene. Uplift took place in Fennoscandia (~600 m, Mrrner 1980), Wales (George 1974) and Scotland (George 1966). If the Lenham shoreline deposits and adjacent sands of the north Weald are of Pliocene age, then there has been relative movement of 183 m between Lenham in Kent and the Coralline Crag of Suffolk (West 1972). The sense of movement is likely to have been principally of uplift in the Weald and this in turn has caused the incision of river valleys to a depth of 100 to 150 m (Jones 1980). For most of the British Isles it is considered likely that the Neogene shoreline was probably close to that seen now. However, it should be noted that this map summarizes events over a period of around 20 Ma during which there were significant eustatic changes of sea level. Only the most generalized picture has therefore been given. It is possible that at times there was communication between the Channel and the North Sea. The North Sea Basin appears to have been the site of mainly mudstone deposition throughout the Neogene. During the Miocene, the Central Graben was the main depocentre (>1300m; Nielsen et al. 1986). The southern shoreline extended into Belgium and the Netherlands and the Neogene deposits here were disturbed by contemporaneous faulting from the Rhine rift system (Pannekoek 1956). The main phase of inversion affecting the Celtic Sea, Bristol Channel, Western Approaches, English Channel and Paris basins occurred during the late Oligocene to early Miocene (Ziegler 1987). In the Western Approaches, the subaerially exposed anticlinorium had structural relief of 3000 m and erosion of Palaeogene and Cretaceous strata took place from the crest. The Miocene carbonate sediments of the Western Channel were termed the Globigerina Silts by Curry et aL 1962. From their content of nannofossils and planktonic foraminifera they have been dated as lower and Middle Miocene (Curry et al. 1965; Jenkins 1977; Martini 1974). In the Zephyr borehole 83/24-1, only the lower part of the Miocene was cored and this consists of green to white, calcareous and glauconitic clays and very fine sandstones (Evans et al. 1981). Murray (in Curry et al. 1965) interpreted the Miocene sea in this area as a gulf extending into the Western Channel with surface temperatures similar to those of the present (i.e. summer, temperature of 16~ Jenkins (1977) suggested slightly higher temperatures of 16-20~ Hughes (in Jenkins & Murray 1981) agreed that there was no seaway through the Channel to the North Sea and he estimated the depth of the Sealab site to have been greater than 400 m. However, these two interpretations are not consistent because such a sea level rise (present depth 128 m; rise ~ 270 m) would surely have flooded much of southern England and northern France and ensured a connection with the North Sea. The St Erth deposits of Cornwall have proved difficult to date but are now considered to be of Pliocene age (Jenkins et al. 1986) and to have been deposited in shallow water within the photic zone and with a temperature of 10-18~ A small outlier of Pliocene clays occurs in the Western Channel (Curry et al. 1965). It rests unconformably on the underlying Miocene. A similar relationship is observed in the Celtic Sea. In the Irish Sea Basin strata of probable Neogene age have been identified on seismic profiles. They rest unconformably on folded Palaeogene and Mesozoic rocks and they can be divided into a lower massive section and an upper bedded section. The former may be non-marine clays and the latter unconsolidated marine sands and clays (Naylor & Mounteney 1975). In the Celtic Sea (borehole 56/14-1, Colin et al. 1981) the Early and Middle Miocene consists of silty shales unconformably overlain by Late Miocene shelly, silty sands which overstep onto Late Cretaceous chalks south of Ireland. The Neogene sediments of the Rockall Trough have not been sampled but are thought to make up most of the post-R4 sequence. From the seismic records it is evident that fans of coarser clastic sediments built out from the slope to the northwest of Ireland. These were fed by canyons running downslope. In the central part of the trough there are turbiditic deposits while along the western margin there are fine-grained drift deposits (Roberts 1975). The latter were formed by a southward flowing bottom current of Norwegian Sea Overflow Water. The Neogene deposits of the Channel and Celtic Sea have been gently folded and there is an unconformity between the Mioceneand Pliocene. The central Channel inversion, including the Isle of Wight monocline and the Weald anticlinorium which have their origins in the Palaeogene, were further developed probably by movement on basement blocks. Uplift of 1.6 km during the Neogene is indicated from the burial graph for Kimmeridge Bay (Stoneley 1982). The compressional regime may have caused dextral movement on the Sticklepath fault (Holloway & Chadwick 1986) and the formation of northwest-southeast-trending mesofractures in southern England (Bevan & Hancock 1986).
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References References not listed here are cited in Curry et al. (1978). BERGER,W.H., VINCENT,E. & THIERSTEIN,H. R. 1981. The Deepp-sea Record: Major Steps in Cenozoic Ocean Evolution. Society of Economic Paleotologists and Mineralogists, Special Publication 32, 489-504. BEVAN,T. G. & HANCOCK,P. L. 1986. A late Cenozoic regional mesofracture system in southern England and northern France. Journal of the Geological Socie~; London, 143, 355-362. BUURMAN,P. 1980. Palaeosols in the Reading Beds (Palaeocene) of Alum Bay, Isle of Wight, U.K. Sedimentology, 27, 121-123. CASTON,V. N. D., DEARNLEY,R., HARRISON,R. K. RUNDLE,C. C. & STYLES,M. T. Olivine-dolerite intrusions in the Fasmet Basin. Journal of the Geological Society, London, 138, 31-46. COLIN, J. P., LEHMAN,R. A. & MORGAN,B. E. 1981. Cretaceous and Late Jurassic biostratigraphy of the North Celtic Sea Basin, offshore Southern Ireland. In: NEALE, J. W. & BRASIER,M. D. (eds) Microfossilsfrom Recent and Fossil Shelf Seas. British Micropalaeontological Society/Ellis Horwood, Chichester, 122155. CURRY, D. 1962. A lower Tertiary outlier in the central Eastern Channel, with notes on the beds surrounding it. Quarterly Journal of the Geological Society, London, 118, 177-205. , ADAMS,C. G., BOULTER,M. C., DILLEY,F. C., EAMES,F. E., FUNNELL,B. M. & WELLS, M. K. 1978. A Correlation of Tertiary Rocks in the British Isles. Geological Society, London, Special Report, 12. , HERSEY, J. B., MARTINI, E. & WHITTARD,W. F. 1965. The geology of the Western Approaches to the English Channel II. Geological Interpretation aided by boomer and sparker records. Philosophical Transactions of the Royal Society, B248, 315-351. EDMONDS, E. A., McKEowN, M. C. & WILLIAMS,M. 1969. British Regional Geology: South-West England. (Drd Edn). HMSO. EDWARDS, R. A. & FRESHNEY, E. C. 1982. The Tertiary sedimentary rocks. In: DURRANCE, E. M. & LAMING,D. J. (eds) The Geology of Devon, University of Exeter, Exeter, 204-237. ELLISON, R. A. 1983. Facies distribution in the Woolwich and Reading Beds of the London Basin, England. Proceedings of the Geologists" Association, 94, 3 i !-319. EVANS, C. D. R., LOTT, G. K. & WARRINGTON,G. 1981. The Zephyr (1977) wells, South- Western Approaches and Western English Channel. Institute of Geological Sciences, Report 81/g, 1-44. FRESHNEY, E. C., EDWARDS,R. A., ISAAC,K. P., WITTE, G., WILKINSON,G. C., BOULTER, M. C. & BAIN, J. A. 1982. A Tertiary basin at Dutson, near Launceston, Cornwall, England. Proceedings of the Geologists" Association, 93, 395-402. GEORGE,T. N. 1966. Geomorphic evolution in Hebridean Scotland. Scottish Journal of Geology, 2, 1-34. -1967. Landform and structure in Ulster. Scottish Journal of Geology, 3, 413418. HOLLOWA,Y, S. & CHADWICK, R. m. 1986. The Sticklepath-Lustleigh fault zone: Tertiary sinistral reactivation of a Variscan dextral strike-slip fault. Journal of the Geological Society, London, 143, 447-452. 1LLING, L. V., & HOBSON, G. D. (eds) 1981. Petroleum Geology of the Continental Shell" of North- West Europe. Heyden, London. ISAAC,K. P. 1981. Tertiary weathering profiles in the plateau deposits of East Devon. Proceedings of the Geologists' Association, 92, 159-168. 1983. Discussion on Eocene sedimentation and tectonics in the Hampshire Basin. Journal of the Geological Society, London, 140, 319-320. JENKINS, D. G. & MURRAY,J. W. 1981. Stratigraphical Atlas of Fossil Foraminifera. British Micropalaeontological Society/Ellis Horwood, Chichester. , WmTTAKER,J. E. & CARLTON,R. 1986. On the age and correlation of the St. Erth Beds, S.W. England, based on planktonic foraminifera. Journal of Micropalaeontology, 5, 93-105. JONES, D. K. C. 1980. The Tertiary evolution of south-east England with particular reference to the Weald. In: JONES, D. K. C. (ed.) The Shaping of Southern England. Institute of British Geographers, Special Publication, 11, i 3-47. KNOX, R. W. O'B. & HARLAND,R. 1979. Stratigraphical relationships of the Early Palaeogene ash-series of N.W. Europe. Journal of the Geological Society, London, 136, 463-470. - - - , MORTON,A. C. & HARLAND,R. 1981. Stratigraphical relationships of Palaeocede Sands in the UK sector of the central North Sea. In: ILLING, L. V. & HOBSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 267-281. MARTINI,E. 1974. Calcareous nannoplankton and the age of the Globigerina Silts of the Western Approaches to the English Channel. Geological Magazine, 111, 303-306. M6RNEg, N. A. 1980. Earth movements, paleoceanography, paleoclimatology and eustacy: major Cenozoic events in the North Atlantic. Geologiskaforeningens i Stockholm forhandlingar, 102, 261-268. 1 9 8 1 .
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MORTON, A. C. 1982a. The provenance and diagenesis of Palaeogene sandstones of southeast England as indicated by heavy mineral analysis. Proceedings of the Geologists" Association, 93, 263-274. 1982b. Heavy minerals of Hampshire Basin Palaeogene strata. Geological Magazine, 119, 463-476. 1982e. Lower Tertiary sand development in Viking Graben, North Sea. Bulletin of the American Association of Petroleum Geologists, 66, 1542-1559. MURRAY, J. W. & WRIGHT, C. A. 1974. Palaeogene Foraminiferida and palaeoecology, Hampshire and Paris Basins and the English Channel. Special Papers in Palaeontology, 14, 1-129. NAYLOR, D. & MOUNTENAY,A. N. 1975. Geology of the North West European Shelf, I, Graham & Trotman, London. NIELSEN, O. B., SORENSEN,S., THIEDE,J. & SKARaO,O. 1986. Cenozoic differential subsidence of North Sea. Bulletin of the American Association of Petroleum Geologists, 70, 276-298. PANNEKOEK,A. J. 1956. Geological History of the Netherlands. Government Printing Office, The Hague. PARKER,J. R. 1975. Lower Tertiary Sand Development in the Central North Sea. In: WOODLAND, A. W. (ed.) Petroleum and the Continental Shelf of North-West Europe. Volume 1: Geology. Applied Science, Barking, 447-452. PEGRUM, R. M., REES, C. & NAYLOR,D. 1975. Geology of the North West European Shelf, 2, The North Sea. Graham & Trotman, London. PLINT, A. G. 1982. Eocene sedimentation and tectonics in the Hampshire Basin. Journal of the Geological Society, London, 139, 249--354. 1983. Facies, environments and sedimentary cycles in the Middle Eocene, Bracklesham Formation of the Hampshire Basin: evidence for global sea-level changes? Sedimentology, 30, 625~53. 1984. A regressive coastal sequence from the Upper Eocene of Hampshire, Southern England. Sedimentology, 31,213-225. POMEROL, C. 1982. The Cenozoic Era, Tertiary and Quaternary. Ellis Horwood, Chichester. RIDD, M. F. 1981. Petroleum geology west of the Sheltands. In: ILLING, L. V. & HOBSON, G. D. (eds) Petroleum Geology of the Continental Shelf of North- West Europe. Heyden, London, 414-425. ROBERTS, D. G. 1975. Marine geology of the Rockall Plateau and Trough. Philosophical Transactions of the Royal Society, A275, 447-509. - - , BACKMAN,J., MORTON,A. C., MURRAY,J. W. & KEEI~, J. B. 1984. Evolution of volcanic rifted margins: synthesis of Leg 81 results on the west margin of Rockall Plateau. In: ROBERTS,D. G., SCHNITKER,D. et aL (eds) Initial Reports of the Deep Sea Drilling Project, 81, 883-911. ROCHOw, K. A. 1981. Seismic stratigraphy of the North Sea 'Palaeocene' deposits. In: ILLXNG,L. V. & HOBSON,G. D. (eds) Petroleum Geology of the Continental Shelf of North- West Europe. Heyden, London, 255-266. SCLATER,J. G. & CHRISTIE,P. A. F. 1980. Continental stretching: an explanation of the post-mid-Cretaceous subsidence of the central North Sea basin. Journal of Geophysical Research, (B7), 3711-3739. STONELEY, R. 1982. The structural development of the Wessex Basin. Journal of the Geological Society, London, 139, 543-554. TALWANI,M. & UDINTSEV,G. 1976. Tectonic synthesis. In: TALWANI,M., UDINTSEV, G. et al., Initial Reports of the Deep Sea Drilling Project, 38, 1213-1242. TUCKER, R. M. & ASTER, G. 1987. The tectonic evolution of the North Celtic Sea and Cardigan Bay basins with special reference to basin inversion. Tectonophysics, 137, 291-307. VAN HOORN, B. 1987. Structural evolution, timing and tectonic style of the Sole Pit inversion. Tectonophysics, 137, 239-284. VAN W,JHE, D. H. 1987. Structural evolution of inverted \basins in the Dutch offshore. Tectonophysics, 137, 171-219. WATSON,J. 1985. Northern Scotland as an Atlantic-North Sea divide. Journal of the Geological Society, London, 142, 221-243. WEST, R. G. 1972. Relative land-sea-level changes in southern-eastern England during the Pleistocene. Philosophical Transactions of the Royal Society, A272, 87-98. WILKINSON, G. C., BAZLEY, R. A. B. & BOULTER, M. C. 1980. The geology and palynology of the Lough Neagh Clays, Northern Ireland. Journal of the Geological Society, London, 137, 65-75. WOODLAND,A. W. 1979. Sub-Pleistocene Geology of the British Isles and the Adjacent Continental Shelf. Map published by the Institute of Geological Sciences, scale 1:2,500,000. ZIEGLER, P. A. 1981. Evolution of sedimentary basins in North-West Europe. In: ILLING, L. V. & HOBSON,G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 3-39. 1982. Geological Atlas of Western and Central Europe. Shell International Petroleum Maatschappij BN. 1987. Late Cretaceous and Cenozoic intra-plate compressional deformations in the Alpine foreland--a geodynamic model. Tectonophysics, 137, 389-420. -1988. Evolution of the Arctic-North Atlantic and the Western Tethys. American Association of Petroleum Geologists, Memoir 43. -
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Quaternary C. C. G R A H A M
The Quaternary is represented widely over land areas and sea floors around Britain by sediments formed under conditions which ranged from warm temperate to glacial, humid to semi-arid, and which involved glacial, periglacial, fluvial, mass movement, marine and aeolian processes. The distribution of Quaternary sediments has been depicted on maps of the Geological Survey since the 1840s and most recently on the Ordnance Survey Quaternary Map of the United Kingdom, (1977, two sheets) at a scale of 1: 625,000. The Oxford Atlas of Britain and Northern Ireland (1963) and the Atlas of Ireland (1979) include Quaternary maps, and Britain is covered by two sheets of the International Quaternary Map of Europe (1967) at a scale of 1:2,500,000. Maps of sea-floor sediments are being published as surveys are completed (Cameron et al. 1987). Stages of the Quaternary sequence as currently established are listed by West (1980) and Bowen et al. (1986).
Qla: Quaternary geography The maximum extent ever reached by icesheets in southern Britain is better known in the east, and the western limit of I~ is an approximation because definitive deposits are sparsely distributed. The Scilly Islands represent the most southerly point reached by a British icesheet, but the limit across the Celtic Sea is hypothetical, drawn with regard both to the nature of the continental shelf and to the fact that Ireland is known to have been wholly glacierized on at least one occasion. This maximum limit may be diachronous. Bowen et al. (1986) regard it as wholly Anglian, but a Wolstonian age has generally been preferred for the limit from the Midlands to the Scilly Islands. In eastern England Wolstonian ice might have penetrated either to the limit suggested by Straw (1979) or no further south than east Lincolnshire (Bowen et al. 1986). Sometill units in the central North Sea area (Sejrup et al. 1987) and the Paviland Moraine in south Wales (Bowen et al. 1987) are considered of Wolstonian age. The most significant river diversion in southern Britain at the maximum stage was the displacement of the Thames from its former course through the Vale of St Albans and the mid-Essex depression. The Warwickshire Avon was initiated as a meltwater stream, and the striking courses of the Avon at Bristol and the Wye in the Forest of Dean may also reflect glacial interference. Most rivers from the Thames and Severn valley northward carried meltwaters during one or other of the glaciations. The furthest limit reached by Devensian icesheets is also a compound feature, and the many recessions and readvances of British and Irish ice lobes have produced complicated stratigraphic sequences and complex morainic zones. Less extensive Devensian ice limits are not shown except for the Loch Lomond Advance in Scotland because of continuing controversy over location, relative importance and date. The maximum limit may be entirely of Late Devensian age, but the possibility that earlyDevensian ice was responsible for parts of it east of Scotland and in eastern England remains. A conservative position for the Vale of York ice is preferred in the absence of tills south of Selby, and the Devensian margin in south Wales seems settled after long controversy (Bowen et al. 1986). In Ireland much of central Munster south of the Wicklow mountains and of Leinster remained free of ice. Scottish ice flowed strongly westward over the Outer Hebrides and adjacent continental shelf for at least part of Late Devensian time calving into the Atlantic along a zone close to the shelf edge parallel to a tract of sea floor furrowed by iceberg plough marks (Lee & Ramster 1981, map 2.27). The Shetlands formed an independent ice centre, and recent work confirms that the Scottish and Scandinavian icesheets were never in contact during the late Devensian glaciation (Sutherland 1984; Cameron et al. 1987; Sejrup et al. 1987). Late Devensian ice is responsible for the present appearance of most glacial landforms in Britain and Ireland. Only the Irish eskers and major Irish and British drumlin fields are shown here. Important river diversions include the Shannon above Limerick, the Liffey through Dublin, the upper Severn through the Ironbridge Gorge and the Derwent along the Vale of Pickering in east Yorkshire. By 13 000 BP, late Devensian ice had largely wasted away with improvement of temperature into the late glacial Interstadial, but by 11 500 BP ice began again to form in Scotland, massively in the western Highlands but less so in the Grampians. This cold period, the Loch Lomond Stadial, persisted for over 1000 years and generated local cirque and valley-glaciers in other upland areas of Scotland, northwestern England, Wales and Ireland (Bowen et aL 1986). The greatest development of periglacial features lies south of the Devensian limit. Periglacial conditions must have preceded, accompanied and 9 1992The GeologicalSociety
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followed each glaciation so that a profusion of materials, landforms and structures survives, but the strongest legacy is Devensian. Although the scale, nature and extent of periglacial activity are being investigated, good general maps are not yet available. During emergent phases, particularly in the Devensian, large areas of present sea floor were substantially affected by periglacial processes. The locations of find-sites of Palaeolithic artefacts are taken from Wymer (1985), Roe (1964) and the Atlas of France (1959) with minor additions. Although some sites have yielded very few artefacts and others many thousands, the pattern still reveals major concentrations in the Breckland region of East Anglia, the Thames basin, central southern England, and the Seine and Somme basins in northern France. The juxtaposition on the map of find-sites with icesheet limits illustrates the probability that glaciation, by destroying much artefactual evidence, could have had as significant a control on artefact distribution as the location and availability of suitable raw materials. AS
Qlb: Quaternary crustal movement Three complementary sets of information provide data on Quaternary crustal movement. Isopachs for Quaternary sediments in the North Sea area are taken from Caston (1977) and Cameron et al. (1987). Below a midQuaternary unconformity sediments in the central and northern North Sea areas are mainly marine shelf materials, but in the southern North Sea are largely deltaic. Above it deposits are generally glacial, glaciomarine and periglacial. The isobases for the Main Lateglacial and Main Postglacial shorelines in Scotland are from Jardine (1982) and reveal the form of the crustal displacement resulting from glacial unloading to be an elongated dome centred on the Western Highlands. Churchill (1965) examined x4C dated deposits formed at sea level 6500 years ago at various sites around England and Wales, and constructed the isobases of relative vertical displacement used here. The sense of crustal development in all three sets of data is therefore similar, confirming elevation in the north and west and subsidence in the east as complex responses to several mechanisms, including loading of the North Sea area during the Flandrian rise of sea level. The interaction of crustal movement and sea-level oscillation through the Quaternary has led to major shifts in coastline location and environments. Although the late Devensian and Flandrian rise of sea level is responsible for flooding many estuaries, rias and sea lochs and for extensive sedimentation over large areas of eastern and southern England, coastal geomorphology is complicated by the survival, particularly around southwest England and the southwestern half of Ireland, of many fragments of raised shoreline features of Ipswichian or earlier date up to 30 m O.D. Relict interglacial shorelines can be identified in eastern England, in particular the old cliff along the east side of the Chalk Wolds of east Yorkshire and Lincolnshire, and raised coastal features recur frequently along the French coast of the English Channel especially around the Cotentin peninsula. Submerged cliffs within the Channel near palaeo-valleys may have been fluvially eroded, some mark contacts of harder/weaker rocks, and yet others may be wave-eroded even to depths of at least 110-120 m below O.D. which probably represents the maximum of pre-Flandrian regression. Numerous closed linear depressions or deeps have been observed on sea floors around Britain. The Hurd Deep in the English Channel is the largest and probably functioned as part of an extensive branching valley system which received French and British tributaries as well as Thames-Rhine and glacial waters from the North Sea area. Groups of deeps southwest of Dogger Bank and off northeast Scotland are likely to be Devensian tunnel-valleys, variously modified by tidal scour and deposition. Seismic profiles provide evidence for many former valleys north of 52~ excavated during glacial episodes within and through Quaternary sediments and subsequently infilled (Cameron et al. 1987). The locations and age determinations of interglacial fossiliferous sites have been taken from standard texts and recent journal reports. Most sites lie in lowland England outside the Devensian glacial limit (which indicates glacial destruction of much evidence) and most Hoxnian sites occur outside postulated Wolstonian limits. Cromerian sites are rare outside East Anglia. In south Wales and southwest England cave locations are significant. Few sites have so far been discovered in Ireland. Although further discoveries of interglacial deposits will undoubtedly be made, gross modifications to the distribution pattern seem unlikely. Recorded Lateglacial Interstadial sites are most numerous in Scotland, north England and eastern Ireland. In Scotland, the sites prove the virtual 149
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disappearance of late Devensian ice by 13 000 BP (Sutherland, 1984), while 14C dated vegetation sequences here and in the Lake District confirm onset of the Loch Lomond Stadial by 11 500 BP. At many sites the lnterstadial deposits are overlain by and therefore allow differentiation of periglacial materials of Lateglacial Zone III (Loch Lomond Stadial) date. The maximum marine regression in the Late Devensian (c. 17000 BP) appears to have been to - 1 1 0 to - 1 2 0 m O.D., with recovery to about - 4 0 m O.D. by approximately 10 000 BP, and to within a few metres of the present level by 6000 BP. The average rate of rise therefore over 11 000 years was around 1 m/100 years and, with gradients of less than 1:2000 over much of what are now the floors of the English Channel and southern North Sea, the rate of shoreline migration in early stages could have been as much as 20 m/year. Such rapid submergence (which probably occurred after each glacial phase) has, by limiting their reworking, ensured both the better preservation of Quaternary sediments beneath the British seas than on land and the possible eventual establishment of a comprehensive Quaternary record. AS
Q2: Present-day seabed sediment distribution and topography
Topography The topography is classified in terms of shelf, slope and deep water, which group together areas of broadly similar morphology and depth rather than tectonic features. On the northwest European continental shelf, the seabed is generally very fiat with average gradients of the order of 1: 3000 to 1: 500. However, in areas of submarine banks, ridges and rock platforms, the topography is more irregular. There are also a number of elongate depressions up to 30 km long and 5 km wide, which extend down from about 20m to 160m below the surrounding seabed. The Norwegian Trench is a large topographic depression about 100 km in width running parallel to the southwest coast of Norway. Water depths within the Trench exceed 200 m in the shallowest parts, increasing to over 400 m to the north. The Trench is deepest in the Skagerrak (beyond the eastern limit of the map), where water depths exceed 700 m. The shelf break is marked by a change in gradient and occurs in water depths varying from about 140m to 250m, although in some areas the position of the major break in slope occurs as deep as 600 m. The slope area beyond the shelf break has gradients of between 1:100 and 1:5, and is steepest south of 59~ The height of the slope varies from less than 1500 m in the north to over 4000 m in the south, where it is heavily incised by canyons. On the Faeroe shelf, gradients are of the order of 1:500, and the shelf break occurs in water depths of between 200 m and 350 m. The tops of Faeroe Bank, Bill Bailey's Bank, Lousy Bank and Rockall Bank have also been shown as shelf areas as they are relatively shallow, fiat areas bounded by steep slopes. The deep water areas shown are the Rockall Trough, the Faeroe-Shetland Channel, the Faeroe Bank Channel and the eastern part of the Iceland Basin. The Rockall Trough is shallowest in the north, with water depths of about 1000m, and deepens southwards to over 2500m at the limit of the map. The Faeroe-Shetland Channel and the Faeroe Bank Channel are relatively narrow features with water depths ranging from 1000m in the south to over 1500 m in the north. Water depths within the Iceland Basin reach 1500 m at the edge of the map.
water mass crosses the Wyville-Thomson Ridge and flows southward along the western side of the Rockall Trough. Wind-induced wave action creates oscillatory water movement at the seabed the intensity of which depends on water depth and the strength, duration and fetch of the wind. Most of the seabed on the shelf around Britain is affected episodically by storm-induced wave action. Storm surges are created when large changes in barometric pressure acting on the sea surface are combined with strong wind drift currents. Surges can generate currents of the same magnitude as tidal currents and when the two are combined they have considerable potential for erosion and transportation of sediment, particularly in the North Sea.
Seabed sediments The sediments at the seabed are a mixture of terrigenous and biogenic material, and their present distribution is the result of a complex and continuing process of erosion, transportation and deposition. They contain a large amount of relict Pleistocene material which has undergone significant reworking during and following the Holocene transgression. Gravelly sediments occur on bathymetric highs, in areas of extensive rock outcrop and in areas of strong currents and wave action. The gravel consists of Quaternary lag deposits, Holocene shell debris and material eroded from underlying bedrock. Biogenic content is often greater than 50%, with concentrations locally exceeding 80%. The gravel deposits are generally only a few centimetres thick, although in areas of gravel bedforms the deposits can be over 1 m thick and may be cross-stratified. Sandy sediments are widespread in the seas around the UK. The sand is mobile and varies in thickness from a few centimetres to sand banks up to 60 m in height. The sand surface is formed into either longitudinal or transverse bedforms, depending on the current regime and the sediment supply. The sand in the North Sea, away from the coastal zone, has a carbonate content of less than 10% and, in many areas is almost noncalcareous. Elsewhere on the shelf, however, the carbonate content is higher and more variable, exceeding 50% in some areas (see Fig. l).
Hydrodynamic environment The sediment distribution is strongly influenced by bottom currents generated by tides, oceanic circulation patterns, wave action and storm surges. The most important factor determining sediment transport on the shelf area around the British Isles is tidal currents. Semi-diurnal tidal waves are generated in the Atlantic Ocean and are amplified when they move onto shelf areas by the decrease in the water depth and resonance effects. Around the British Isles, near-surface peak tidal currents are generally within the range of 0.5 m s - 1 to 1.0 m s - 1 and maximum values locally exceed 4 msThe highest tidal current velocities occur in the Irish Sea, the English Channel and around Orkney and Shetland. In the North Sea, the tidal flow is anticlockwise, southward along the east coast of Britain and northward along the coast of Denmark and Norway. Current speeds in the west are generally greater than 0.50ms -1 and increase to over 0.75ms -1 in the south. In the northeastern North Sea tidal currents are less than 0.25 ms- 1. Oceanic currents result from the flow of saline surface water from the Atlantic into the Norwegian Sea, which is balanced by a deep return flow of cooler, more dense water. The surface flow reaches depths below 400 m and creates a strong, northerly flowing current on the slope off northwest Europe where peak current speeds probably exceed 0.75 ms-1. The current sweeps onto the shelf north of the Wyville-Thomson Ridge, part of it flowing into the North Sea through the Orkney-Shetland Channel. To the north of Shetland, the Atlantic water diverges, part continuing northwards into the Norwegian Sea and part flowing southwards into the Norwegian Trench. The colder deep water current returns southwards along the FaeroeShetland Channel, with the core of the water mass below 800 m. Some of the
Fig. 1. Muddy sediments on the shelf tend to occur where tidal currents and wave action are weak, near the mouths of large rivers, in bathymetric lows, or in areas where the substrate has a high mud content. The mud can be tens of metres thick. The carbonate content of the mud on the shelf varies from about 10% to over 40%, and is largely comminuted shell material. In the
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northern North Sea blocks of carbonate cemented mud occur in pockmarks as a result of enhanced biological activity, possibly resulting from bacteria feeding on methane gas within the sediments. In deep water areas beyond the shelf, calcareous mud is predominant. Sediment transport The main sand transport paths around the British Isles are aligned parallel to the directions of the maximum tidal currents. Locally, sand transport paths can vary from the regional trends due to local tidal gyres and non-tidal currents. Some sand is also being transported inshore from the open shelf, and in the southwest some sand is being lost from the shelf on to the slope. On the upper slope west of Scotland, the transport paths are aligned with the northerly flowing slope current. Bedforms Transverse and longitudinal bedforms associated with sediment transport are widespread features on the shelf and upper slope. Most patches of sand on the shelf are covered by ripples. Where tidal currents increase in strength sandwaves occur either as isolated features or grouped in fields on rock and gravel pavements, on sand sheets, and on the flanks of tidal sand ridges. Longitudinal bedforms on the shelf include sand ribbons, gravel furrows, longitudinal sand patches and tidal sand ridges. Contourite deposits occur on the slope areas. The tidal sand ridges in the southern North Sea and Irish Sea are up to 50 km in length, 20 km to 30 km in width and 40 m in height with a separation of between 5 km and 10 km. The shallower ridges have sandwave fields on their flanks suggesting continued modification by the currents, while the deeper ridges tend to lack sandwaves and are probably moribund. The largest sand ridges on the shelf occur in the outer Celtic Sea where they are up to 200 km in length and 60 m in height and have a separation of between 10km and 15km. CCG
Sources The information on topography and sediment distribution is based mainly on a number of published maps by research groups in the U K (British Geological Survey, Institute of Oceanographic Sciences), France (Bureau de Recherches Grologiques) and Norway (Continental Shelf Institute). Information relating to various aspects of the topography, the hydrodynamic environment and the sediments was obtained from a number of published accounts (Roberts et al. 1979; Stride 1982; Kenyon 1986; Pantin 1991).
References Bathymetry of the Northeast Atlantic, Sheet 2. Continental Margin around the British Isles. I : 2,400,000. Institute of Oceanographic Sciences, UK. BOWEN, D. Q., ROSE, J. & McCABE, A. M. 1987. Correlation of Quaternary glaciations in England, Ireland, Scotland and Wales. In: SmRAvA,V., BOWEN, D. Q. & RICI-IMOND,G. M. (eds) Quaternary Glaciations in the Northern Hemisphere, Quaternary Science Reviews, 5, 299-340. Carte des Sediments Superficiels de la Manche. 1:500,000. Bureau de Recherches Grologiques et Mini~res, Service Grologique National, France. CAMERON,T. D. J., STOKER,M. S. & LONG, D. 1987. The history of Quaternary sedimentation in the UK sector of the North Sea basin. Journal of the Geological Society, London, 144, 43-58. CASTON,V. N. D. 1977. A new isopachyte map of the Quaternary of the North Sea. Report of the Institute of Geological Sciences, 77]11, 1-8. CHURCHILL,D. M. 1965. The displacement of deposits formed at sea-level 6500 years ago in southern Britain. Quaternaria, 7, 239-249. JARD1NE,W. G. 1982. Sea-level changes in Scotland during the last 18000 years. Proceedings of the Geologists" Association, 93, 25-41. KENYON, N. H. 1986. Evidence from bedforms for a strong poleward current along the upper continental slope of northwest Europe. Mar. Geol., 72, 187-198. LEE, A. J. & RAMSTER,J. E. (eds) 1981. Atlas of the Seas around the British Isles. Ministry of Agriculture, Fisheries and Food, London. PANT1N, H. M. 1991. The sea-bed sediments around the United Kingdom: their bathymetric and physical environment, grain size, mineral composition, and associated bed-forms. British Geological Survey Research Report SD/90/I. Quaternary Geology: 60~176 I~176 1:500,000. Continental Shelf Institute, Norway. 1984. ROnERTS, D. G., HUNTER, P. M. & LAUG,TON, A. S. 1979. Bathymetry of the northeast Atlantic: continental margin around the British Isles. Deep Sea Research. 26A, 417--428. ROE, D. A. 1964. The British Lower and Middle Palaoalithic: some problems, methods of study and preliminary results. Proceedings of the Prehistoric Society, 30, 245-267. Sea Bed Sediments around the United Kingdom. North and South Sheets. 1: 1,000,000 Special Map Series. British Geological Survey, UK. 1987. SEJRUP, H. P., AARSETH,I., ELLINGSEN,K. L., REla'rm'g,E., JANS~'N,E., LOVLIE,R., BENT, g., BRIGHAM-GRETTE,J., LARSEN,E. &: STOKER, M. 1987. Quaternary stratigraphy of the Fladen area, central North Sea: a multidisciplinary study. Journal of Quaternary Science, 2, 35-58. STRAW, A. 1979. The geomorphological significance of the Wolstonian glaciation of eastern England. Transactions of the Institute of British Geographers, 4, (New Series), 540-549. STRIDE, A. H. (ed.) 1982. Offshore Tidal Sands. Chapman & Hall, London. SUTHERLAND,D. G. 1984. The Quaternary deposits and landforms of Scotland and the neighbouring shelves: a review. Quaternary Science Reviews, 3, 157-254. WEST, R. G. 1980. The Pre-glacial Pleistocene of the Norfolk and Suffolk Coasts. Cambridge University Press, Cambridge. WYMER,J. J. 1985. The Palaeolithic Sites of East Anglia. Geobooks, Norwich.
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